Mega Delphox having Levitate is a perfectly fine ability. Turning a weakness into an immunity is always gonna be good, even if it can’t spam Expanding Force. It’d arguably be overpowered if it did get Psychic Surge or something. It’d be Mega Charizard Y but faster and immunity to priority.
Mega Chesnaught definitely got the worst end in terms of abilities. It could have gotten Stamina or something, but instead its just bulletproof, which is fine, but its a slow defensive Grass/Fighting type. With stats similar to Mega Golisopod who has vastly superior typing too.
As for Mega Raichu X, I don’t think it’ll actually get Levitate. They didn’t show off Raichu facing a ground type or anything. It’s more likely now that Raichu X will get a new ability that raises its attack after using Volt Tackle.
"It’d arguably be overpowered if it did get Psychic Surge"
No, it would undeniably be overpowered. Thats a 1.5 * 1.3 * 1.5 (EForce boost, Terrain boost, STAB for being psychic). That turns Expanding Force into a 234 base power move.
252+ SpA Delphox-Mega Expanding Force (120 BP) vs. 252 HP / 0 SpD Assault Vest Raging Bolt in Psychic Terrain: 238-282 (52.4 - 62.1%) -- guaranteed 2HKO.
(guranteed 3hko in doubles)
252+ SpA Delphox-Mega Expanding Force (120 BP) vs. 248 HP / 0 SpD Ursaluna in Psychic Terrain: 391-462 (84.4 - 99.7%) -- 37.5% chance to OHKO after burn damage (Singles)
252+ SpA Delphox-Mega Expanding Force (120 BP) vs. 248 HP / 0 SpD Ursaluna in Psychic Terrain: 294-346 (63.4 - 74.7%) -- guaranteed 2HKO after burn damage (Doubles)
Bolt need either additional EV's into spdef or a +spdef nature to have a chance of a 2hko, and doing that much damage to Ursaluna with a neutral+unboosted attack could be classified as a war crime given that Mega Delphox is 134 base speed, one point below Flutter Mane. Don't forget it gets Nasty Plot too so one NP later its wiping the enemy team off the face of the planet.
"just use a steel type to resist the attack"
they click flamethrower and your steel type gets dustbinned. gg. I guess some steel types like Bronzong have a way to alleviate their Fire weakness, but those steel types are typically weaker then the strong steel types anyway.
"just use a dark type to be immune to the attack"
If the opponent doesn't need to run Focus Blast because they know how to teambuild and will inevitably bring a fighting type, usually Urshifu, they'll probably pack dazzling gleam, which gets followed up with a Close Combat that you cant do shit about thanks to Unseen Fist. And on the off chance you're neutral to Dark or Water or even worse weak to Water, now you cant even use screens, intimidate, or defense boosts to stop Surging Strikes/Wicked Blow thanks to the guranteed crits.
"just replace the terrain"
Theres a few issues with this. Mainly that Rillaboom, the next most common terrain setter, speed ties Indeedee-F, the (usually) better one, as Indedee-M is 10 points faster at base 95. Pincurchin is 15 base speed and does nothing other then setup electric terrain. And Galarian Weezing always prefers to run Neutralising Gas.
As for the Tapu's, only Bulu can reliably setup its Grassy Terrain because its the slowest of the 4, at base 75. But running Tapu Bulu means you arent running Rillaboom, Amonguss, Ogerpon, or you're running 2 grass types, which is typically a bad idea unless your team specifically requires it. It doesn't help that Tapu Bulu's currently best fairy stab is Dazzliing Gleam.
Theres always a few caviates to this. Champions could potentially nerf Urshifu when it becomes available, Tapu Bulu could be given good physical fairy STAB, some of the Gen 10 mons might be able to hard counter it, etc etc. But thats all fictional theorymonning.
TLDR: Mega Delphox would absolutely be overpowered with anything other then Levitate. Be glad Game Freak realised this.