Flareon for Ultra League
My only thought on the eeveelutions in general is ALL of their charge moves share their respective typing.
You may swap into a Slowbro with your Meganium and, despite your type advantage, will have a decision when it comes to shielding. Are his two moves Water Pulse and Psychic? Or is he about to slam you with ice beam?
If you're fighting against a Flareon, the question is "is he going to hit me with his fire-type charge move, or his fire-type charge move?"
I thought about this too, a lot of Pokemon with mono-type charge moves will get completely walled pretty easily. It's amusing that their promo art features Electivire at the center of the clash considering it get gimped by this.
Maybe when PvP does roll out we'll see moveset expansions?
We don't know the exact mechanics yet, so absolutely nothing is worth powering up yet. Make decisions after its out or they release all data about the mechanics down to details.
But like others said, most Eeveelutions are unlikely to be that good due to having no coverage moves.
Certainly not yet. We need to know more about PvP. One catch might be that pokemon with two (or more) good charged moves get a big boost. For example, contrast this Flareon with a Houndoom, which could have either Snarl or Fire Fang, and have both Fire Blast and Foul Play. The fire moves are weaker, but a strong Dark move with STAB might make Houndoom a better overall contributor to your team of three.
My advice for PvP now - save anything you think might be useful, until we know more. Don't invest candy (especially RC) or dust into anything.
Okay, one exception. If you have a 96%+ meta-relevant pokemon that will power up to 1490-1500 or 2490-2500, you can put some dust into that, knowing that if PvP doesn't work for it in that league, you could power it up completely and get use out of it for raids and the Master league.
Another exception - if you have a limited window to get a special move (like Community Weekend) and think the pokemon will be good in PvP with that move, get it, but don't further power up or evolve until we know the full story with PvP.