Dota 2 is absolutely unplayable above 150 ms. Last-hitting and denying get hard enough, but at an even higher level you MUST be able to react to in-air projectiles with abilities such as Black King Bar, Manta Style, Blink, Naga's Mirror Image, etc to disjoint the projectiles.

Anecdotal evidence: historically I have a 60% winrate on US West servers (~40-70 ping) and only 50% winrate on US East servers (~110 ping). I've played on EU servers and it is absolutely miserable at 200+ ping. If I had to play regularly on anything above 110 and I'd probably drop to 45% or 40% winrate.