I'm afraid I cannot contribute anything useful regarding the suitability for a motherboard. However, I have been using ultrasonic cleaners for 35 years or so.
The ultrasonic power is important, and in particular in relation to the volume of the fluid. Some low cost cleaners use the same circuit board and transducers over two or even three different sized models. 50W of ultrasonic power in a small tank can punch holes in metal foils, but in a larger tank can be insufficient to do a decent clean.
I've never heard of mixing water and alcohol for use in an ultrasonic cleaner, but I must emphasise again that I haven't used an ultrasonic on electronic boards. In the early days I used trichloroethane 1.1.1, which worked brilliantly, but of course every time you uncapped the bottle you wiped out another cubic mile of ozone (this is an exaggeration, but you know what I mean!). Nowadays most ultrasonic cleaning fluids seem to be aqueous: surfactants plus water, although those intended for use on metal parts usually have a hydrocarbon base.
In terms of cleaning time, I often let the machine run for up to 30 minutes. There is a reason: if your cleaner does not have a heating element then the longer you run the cleaner the hotter the fluid gets, and that always (in my experience) helps the cleaning action. A heating element is definitely worth paying a bit extra for, in my view.
I realise you know this, but I want to emphasise it: the transducers are affixed to the bottom of the tank by a special cement which is glass-hard, in order that it doesn't absorb any energy from the transducers. It is therefore very important NOT to pour hot water into the tank, because the sudden differential expansion CAN cause the cement to crack. Not always, but yes, it has happened to me.
Nearly all cleaners use a swept frequency, but usually swept at the mains electricity frequency which I suspect is too fast. I modified one of my cleaners to sweep much more slowly, and I think it is better because it allows the vibration patterns to build up in the fluid. Some cleaners offer a manual control over the frequency (you should set it for maximum agitation), and others have a closed loop system where they automatically find the frequency at which maximum energy transfer from the transducers to the fluid takes place.