Turby:
Haven't had any problems with finishes. I use MPro7 as my cleaning solution and it is safe for just about everything out there, including plastics and paint.
Some of the guns I've used my ultrasonic on are HK's (MP5, USP series, P7 series, P9 series, Nickel P7, etc), Beretta (Inox and standard finish, Billenium), SIG's in both nickel and the two types of black finishes they have (can't remember the names off the top of my head), S&W revolvers blued, stainless, and nickel.
I think a lot of it has to do with the solution and the tanks. I find the best way to use an ultrasonic and NOT harm the finish is to use wire supports or something to hang the item/gun to be cleaned so that it does not touch the bottom and sides of your subtanks. I'm comfortable enough with the effectiveness of the ultrasonic that I even use it on the more costly pre-bans I've owned (SIG rifles, AUG's, etc) and my machineguns.
Also, you can leave Trijicon night sights on the guns, but I don't believe other brands will honor their warranty if you use an ultrasonic for cleaning.