Here are all ways they could have made Ultimate forms actually interesting and not shitty gimmicks:
Time Limits. Ben can already turn into whatever alien he wants whenever so its not really special when he turns into anything unless its something new that we haven't seen before. So when he turns into the ULTIMATE alien, it should be reserved for absolute emergencies. Ultimate forms aren't reliable, maybe they take a lot out of the watch whenever he turns into them. Again, take a note from the OS when it comes to recharge rates. Maybe the longer he stays in Ultimate form the longer he has to wait until he turns into another alien.
Ultimate Alien Personalties. Ultimate forms, despite being the final stage of evolution or whatever the hell that means, they all look really barbaric. All of them look like animals or monsters than they do aliens. So how about Ben has to deal with new primal voices inside his head the longer he goes ultimate? Each would be different for each alien, Bigchill shares no compassion for anyone, Humongosaur is a giant angry dinosaur, Spidermonkey is a literal gorilla, etc. The longer he stays in these forms the more he risks these new personalities overtaking his own. Kind of like Ghostfreak in the OS
Overpower Overkill. Make his new ultimate alien powers too strong for him to handle. He should be out of control in these ultimate forms as he's dealing with a new level of power he's never had to use before. Ultimate Cannonbolt may be ten times stronger than regular Cannonbolt, but what if Ultimate Cannonbolt can't stop rolling once he's started? Give him a really good cost to whatever power he gains once again making it so that whenever he needs to go Ultimate you know that shit's really going down.