I haven’t always liked it either, but I’ve found over the years I like that “slow” mechanic. Why? Because it’s a unique stat that needs resistance. I feel Grim Dawn as a base succeeds because there’s alot of ways to interact. Slow debuff is an incredibly powerful thing to have for some characters!
Oh yes, bosses being immune to different effects. That sucks. 
But you still slowed the other monsters before hand right? 
It’s similar to having fire as your main damage and facing a fire immune monster. Without it the game would be easy. Annoying I agree, even deadly, but also a more interesting game as a result. I find it useful that some builds kill some bosses easily and other cannot. Look for those you can exploit unless you absolutely have to pass a quest boss, which might take an hour with a build that’s naturally countered by it.
Duriel really got my summons once. But then I realized without that encounter I’d have steamrolled everything. That’s how I feel about skill immunity in some bosses. Think about how to defeat him. 
Am a bit slow today, you said it’s useless if it can only be used on normal monsters. But that’s what you are killing 90% of the time, or did I miss something?
PS, a secondary thing here: I installed a Ryzen 5600 on my existing motherboard. It’s the biggest upgrade since my 1080. Makes a MASSIVE difference in peformance with Grim Dawn for a player who enjoys DSR and max AA alot. Grim Dawn is largely CPU bottlenecked unless you have the latest and greatest.
The new CPU makes a bigger difference than adding an SSD a few years ago. Think about that. I also now have newer 1TB M.2 SSD and that had only minor differences (or no differences, literally).
I’ve been stunned by this, as my old CPU was an overclocking king.