I tried it years ago, and it didn't work out well. I had a 150 watt unit. I found that you need a bigassed unit to get the garbage off - maybe 1,000 watts.
I DO polish cases in a BIG Dillon, with corncob and either Midway or Dillon polish - I put them in and let it run over night with a timer - in the morning the cases come out looking like they were polished on a jeweler's wheel, and gold plated.
For really funky cases with green verdigris, like bulk military, I wash them in Iosso case cleaner for a few minutes, then dry them with a heat gun, and then toss them in the Dillon polisher over night.
I tried ultrasonic a few years ago (as a test of course) and now once again I use my corncob media and a tumbler. The up side is that I have a fine ultrasonic cleaner taking up space on the countertop, it makes a good ring and watchband cleaner for my wife (she used it ONCE... much like me).
I got 2 ultrasound cleaners. They both work great at cleaning. I also have a RCBS tumbler. I use my ultrasound cleaners more on cleaning my dies and fishing gear. My cleaners are small and can't do mass quantity versus what I can put in my tumbler. Also it takes a good 24 hours to clean using the ultrasound.
I have been using Ultra Sonic cleaning for about two years with no problems what so ever. After much researching, the folks that bought the less powerful units have had problems. I got the one for $399.00 from Brownell's. I know a lot of money but well worth it. My routine is to take fired brass & use a universal de-primer and then put the brass neck down in a 1000ml glass lab beaker. The solution is 1/2 water & 1/2 white vinegar from Sam's Club with about three drops of Dawn dish soap. 1/2 - 1 hour & the brass comes out with no fouling in the neck, primer pocket, or interior. I then put the brass in a baggy with some liquid RCBS resizing lube squirted in the baggy. Mix brass around and then resize. I then use the RCBS water case cleaner to polish the brass. You can save some time by running Birchwood Casey liquid case cleaner. Jason Baney is the one who should get the credit for this time saving procedure of the Ultra Sonic cleaning. While it may take a little extra time. You get very clean brass inside and out.