I do believe they offer upgrades to the gen 3 controllers which are VESC based on not ASI although I don't know that the problem is the controller exactly. The ASI controllers are quite good relatively speaking, the issues are lack of access to settings and the display/controller paired with them. If your complaint is not being able to get the controls to respond how you would like that's really CYC's fault for how their display program works how they tuned the BAC so depending how much they've improved it will determine if the new controllers is better. Now I don't think ASI is totally not at fault, if they made them easier to program we would have a lot more options.
I guess I would ask why do you want to replace the controller?
My ASI BAC855 died just after 6 months of use (~2000km) in my STEALTH GEN2. So yes ASI sucks.
It was mildly used (pedal assist only with no more than 500W)
Theoretically you can upgrade to x6 controller but CYC still does not have them in stock. I'm still waiting for my x6 since 1st of march.