I end up feeling like Meganium-Mega just has worse Drought, which they were already comfortable giving to a Mega. In a weather face-off, it would still get its preferred weather active except when fighting TTar-Mega specifically because of when Mega activates, and it loses any team-support potential the field condition provides. I don't feel like going from a 3* fire weakness to a 2* is worth it. The concept has some fun potential for a dual-weather set, but Meganium doesn't have the movepool for it.
I don't get how Drought is a better ability. The only thing Drought has for it over Mega Sol is Drought allows Meganium to be your Sun-Setter, so you can opt to use it over Torkoal or Nines if you are going for that team archetype, but if you are using Mega Meganium as a standalone mon in other team structures, it is literally better Drought.
It doesn't care about Politoed, Alolan-N, T-tar or whoever switching in. You don't have to predict anything before firing a STAB Solar Beam from 140+ base SpA because all those 3 get utterly rekt and OHKO/2HKO on the spot. Drought cannot allow to do that and has to use Energy Ball or you get yourself into a terrible position when the Sand/Hail/Rain overrides your Sun in the turn you click Solar Beam. Your healing is also non-weather related so you always get the hyperhealing from boosted Synthesis and your Meganium shitting on Scizor and other steels doesn't rely on the weather (aka coverage-in-a-timer).
You could, also, run this on Rain teams where Fire attacks are weakened while you keep using those moves as if Sun was going but on Rain instead. Aside from using Meganium as your sun setter, sun doesn't provide anything that Mega Sol doesn't already but better.
. Yes, not being able to support the rest of the team isn't great
Not setting Sun =/= not supporting the rest of the team. Sun is absolutely irrelevant in terms of your progression/team support unless your team archetype is based on said weather (be it fully or partially, fully meaning standard Sun team and partially because maybe your offensive core has some shit that spams specs fire stab like idk, tera fire heatran, while not being a sun team).
Meganium has great utility moves like Knock Off, Screens, phazing options in tail, that actually help the team a lot. Mega Sol doesn't mean you have to go for 252 SpA Evs Modest, it just means that your Meganium, even your support Meganium, has boosted Synthesis and 120 BP STAB 1-turn-charge move even when uninvested.