It's true what you say mr. Lol. Support Dialga and TTar are two threats Espeon's teammates will have to deal with it, support Dialga is especially hard to stop. Espeon can't be thrown into any team because it needs teammates to directly threaten spikers and SR setters, because it won't be able to stop hazards on its own.
But, really, it doesn't come down to that much prediction as you may think. How many times do you get to set up SR with a pokemon that has no reliable recovery? Maybe once or twice against the offensive pressure you are facing when playing a team with Espeon. Hidden power fighting puts Dialga in KO range of many things, Draco Meteor from specs, Kyurem, and scarf Resh's Blue Flare. Usually you want to use Espeon on a sun team, simply because they can threaten Ferrothorn among other important slow spikers way better than rain teams. Rain+Espeon is ok, but you have to resort to way fewer ways to OHKO Ferrothorn and Dialga (LO Terrak, Specs Kyurem-W are alternatives, but list isn't too long).