To jump in for a second, the advantage is having most of the DC universe at play.
I mean I get that there are some limitations... though a version of Superman exists in Supergirl.. and of course Bruce is on Gotham.
I think people look at what they are missing (and some of it I think is stupid), I mean The Trickster is a Flash villain, it makes a lot more sense to go with him than the Joker who realy has no relation to The Flash.
But looking at The Flash and Arrow and I guess Supergirl. Where we've gotten most of the Flash's rogues, we've had Deathstroke, Ra's al Ghul, we have had the Suicide Squad, King Shark, Gorilla Grodd, Anarky, Red Tornado, soon Toyman, Livewire and the list goes on... is still better than
We've had The Atom, Huntress, Black Canary, Vibe, Martian Manhunter, Arsenal, Amanda Waller, Katanna, Firestorm, etc etc
I mean yes they don't get EVERY toy in the chest, but clearly playing with a lot more toys than most MCU shows are allowed.