I haven't looked into ultrasonic cleaners in a while, but I have a couple of friends that do. They are on the mainland, so can't try out.
Anyways, a big part of cleaning IMO is the opportunity to monitor the wear on parts and evaluate if they need to be maintained or replaced. Wear patterns can tell you quite a bit about mechanical devices.
Personally, I want to clean my guns if I won't shoot them for a while. But I'll also admit that there are times when I have an idea to shoot, X, Y, Z, etc guns on a given range day and only end up shooting a couple because I'd have to clean them. I mean I could just not clean them, but personal preference.