Those Megas are brought up (specifically in #future-submissions) quite a bit, and i am going to say from the start that while they can be incredibly cool,
it's generally a bad idea.
First of all, form changing is one of the Megas categories where i personally consider the amount of flavor needed is the highest. If you look at canon Pokemon with form changing abilities, almost all their flavor is based around it (except Darmanitan but that one is made to be a funny contrast of having half the flavor be "angry" and the other be "calm").
Secondly, having a form change be mechanically interesting can be a challenge, and a form change for the sake of it will just feel forced. Let's take a look at some examples to see what i mean:
-First of all,
inkbug's SOS

Wishiwashi. The ability gives it more stats as time passes on. From the mechanic that triggers the form change alone, this one encourages a stally playstyle with moves like Protect to gain turns. The stats follow the same logic: It may hit hard in School form, but it's also extremely bulky and has the right tools to work defensively (including a powerful STAB Scald, even uninvested). Finally the new moves, specifically Wish, further enforce that playstyle by giving it good tools for the longevity that ability requires to work to it's fullest.
That Mega (which has a really solid flavor especially since Wishiwashi
already has multiple forms before Mega-Evolving) takes a new way to change form and by thinking about what it does and what using such an ability requires, create a unique playstyle that merges perfectly into it's stats and move additions as well. (And if you're still not convinced, Wishiwashi's pure Water typing by itself already encourages defense more than offense.)
-Secondly,
ausma's Stance Change

Falinks. Here, the trigger is completely different: Instead of activating as time passes, it activates when using one specific move for one form and attacking moves for the other: Already, this has a totally different implication from Ink's Wishiwashi. While the latter was a one-time change which in return made the Mega stronger as time went on to make it worth it, this one can change between the two freely to take advantage of their unique properties. Ausma took a very interesting approach here, which all comes back to the trigger: One form is entered when using an offensive move, the other when using King's Shield, a priority move with a Protect effect. Here, the one that needs the more power is clearly the one that will actually attack, but there's one big problem with just making "slow defensive form, fast offensive form" here: Stance Change activates when you move and not at the start of the turn. While that mean that the defensive form is very easy to access, it also means that if the defensive form is too slow, the higher speed of the offensive one won't matter as much because there will be a turn in which you will still be slow and take a hit. Ausma did exactly the opposite to counteract this, by makig the defensive form fast, and making the offensive form much slower: While this would also come with the problem of having to take a hit in the frailer, offensive one, ausma gave it incredible offenses, which often makes it unnecessary because you will most likely KO the opponent. Then, once you are in offensive form, you can get back to defensive freely by using King's Shield, and be fast again. Having a defensive form also allows for a great ease of set-up, so the offensive form being slow helps keeping that in check.
Overall, it's a great example of a Mega that takes advantage of actually changing form by thinking deeply about what triggers the change and what that implies.
-Finally for good examples, my own Sundown Switch

Cacturne (while it didn't win, it's still a good example and is still widely liked today). Sundown Switch was a totally new ability, based on Cacturne's well-known flavor of finishing of travelers exhausted by the desert's heat during the night. It played out like this: "When using a Grass type move, changes to Day form. When using a Dark type move, changes to Night form.". Here, what needed to be thought about isn't the trigger itself but what kind of moves it entails: For Grass moves, it includes moves such as Spiky Shield, Synthesis or Leech Seed, while Dark moves included Sucker Punch or Pursuit (it didn't already have Sucker Punch but i gave it). Thus, i made the Grass form be defensive and the Dark one be offensive with the defined role of the Grass one chipping down the opponent with Spiky Shield, Leech Seed and the likes while the faster and stronger Dark one could finish them with it's high Attack stat and STAB Sucker Punch and Pursuit. While i could've stopped here, as it already was interesting, i decided to make one last step in order to make the Grass form better while giving it new tools for both forms: Instead of keeping the defensive form Grass/Dark type, i changed it to Grass/Ground (let's face it, Grass/Dark is a really bad typing). While this did fit on Cacturne because of it's relation to deserts, i also used this as a justification to give it two new options: Earthquake and Scorching Sands. The former gave Night form a strong coverage option, while the latter allowed Day form to further be annoying to the opponent and chip it. Oh and it also stayed in Day form when under Sun, mostly for flavor but also because it plays well with Synthesis, and because Grass types are strongly connected to Sun already
This one was an interesting one due to having a new trigger and making both forms fit the kind of moves that included to strongly fit both of Cacturne's types and it's flavor. .
TL;DR on those three: They were interesting due to fitting the flavor of their respective mon, as well as thinking deeply into what activated the changes to create a unique playstyle and take advantage of the form change.
To show how a Mega like this can be done wrong i'm going to make up a few examples:
-Let's imagine Mega whose ability would change it's form depending on it's highest stat, considering the raw stat. That would mean that there's absolutely no way to change it after the battle starts, and thus that it's not functionally not different from having to choose between 6 Megas in the teambuilder. That is a very important distinction to make:
If your Mega cannot change form after the battle actually started, it's not a form changing Mega. It's multiple Megas for the same mon. Now, let's say that we take that ability again, but change it to take stats changes into account. That would make it mechanically interesting depending on it's stats, right? Well yes, but here, there are still two problems here: For one, this is a very versatile Mega, which would probably be very awkward and hard to play against if you don't know it's set. That is a mistake that can happen very easily:
Your Mega may change form, but it shouldn't have wildly varying counterplay between forms (or at least, not if it can change between them easily). For example, Mega-Wishiwashi may have varying stats between it's different stages, but it keeps the same counterplay. Falinks, while still being very threatening, has some counterplay for it's two forms: Pokemon that are faster than it's defensive form and can OHKO, or Pokemon that can take a hit and KO back. That is a problem that
will appear if you give too many forms to a Pokemon that can change between them as it wants. Something like Wishiwashi can get away because the forms always appear in the same order, and because counterplay to it's School form also will always work for the ones before it.
Secondly, having too much forms like that will also make your Mega very unfocused and unclear, which will also probably make it very weird to build with, especially for the investments.
Now, for the final point i'd like to bring up, let's imagine you got an extremely interesting ability, that is well thought out and has stats that work with ow the form change works, encourages a unique playstyle that gives a reason to why it would have multiple forms. This final step should be the first one you think about before even starting the stats or the ability:
Why would the Pokemon you're giving this form change to have this ability? What part of it's flavor shows that it would get a form change? I know i already talked about that, but flavor is really important for this kind of Mega. Wishiwashi already has all the necessary flavor since it's base form already has one, Fallinks is based on a military group, so it makes sense that it would be able to change formation, Cacturne has the flavor of taking advantage of the day exhausting travelers to finish them off at night, Ice Face Vanniluxe makes sense for reasons i don't even have to explain...
If you look at all previous popular M4A subs that included a form change, you'll notice they all are stongly connected to their respective Pokemon's flavor, much more so than any other kind of abilities (even field effects).
This concludes what i wanted to say about form changing Megas.