In main SB's nuker skill roadmap (Fallen Arrow-> Abyssal Blaze -> Soul of Pain) i see pretty bad thing: after upgrading from Abyssal Blaze to Soul of Pain (on lvl 40) i see that new inproved skill have a longer cooldown time (and a little slower casting speed maybe) than older skills. Its the first time i see a new skill being not only improved, but nerfed too.
Is it a bug in skill system or made on some strange purpose?
Soul of Pain is a different skill from Abyssal Blaze, I wouldn't call it "improved AB", but whatever.
Soul Breakers aren't nukers. As I can immagine this was done to prevent pure nuker setups on Soul Breakers. Also take notice, that Soul of Pain has a chance to cancel enemies target status (quite high too) and does very high damage as for melee character. It would be somewhat overpowered to let it refresh fast enough to chain it non-stop.
It's, ofcourse, only my opinion, but I haven't heard it should be different from what it is now.
For AB you can't spam it non-stop like a real nuker too, but cooldown time is good for nuking every mob and finish it with melee attaks. But with SP i need to wait a bit before i can nuke other mob. Difference is not that much, but perfect to crash non-stop exp chain :)