Limited uses. On an item that can be upgraded for more charges and for which your golem will pay for by itself. One hero? Diffusal is core on more than a few heroes and is situational on almost any agi, especially if you see warlock on the other team. I'm curious as to which one hero you were talking about. The two most likely candidates are both perfectly able to deal with warlock. Riki has a blink and silence, and PL has lance.
You're right, 84% is a massive slow. Maybe it's a pro that you only need 79% to slow someone with max ms to min ms. Either way, it's channeled, on a squishy hero, who will pretty much never get bkb. Why not stun or silence him if he's such a threat? There are plenty of ranged stuns and silences in the game and that's all you need to make him worthless after he pops his ult.
Depend on your enemy to not counter your warlock pick. Depend on them to not build a pipe or bkbs or blink. He's a hero who depends on the other team being actively bad and refusing to counter him. Like brood, the only way he excels is when his opponents are lazy and/or bad.
So... I'll take him instead of.. who exactly? What role does he fill? He can't do a carry caster thing like tiny, dp, qop, or lesh can. He has good damage... every 3 minutes. Compare him to other wombo heroes. I was listing heroes... but it's just so monotonous. Easily half the supports counter his slow. Easily half of the supports have comparable wombo with utility that are off cd more than once a minute.
You think a hero is great because he has a niche. There's a very strong argument that you're wrong. I'm sorry if you find that offensive. I really wish he was better, I really do love his wombo.
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Congrats! There're a couple of skeletony guys. Most of the time you want your stun first.