I have the UL350iS installed on my RV-12. I just finished my second annual with over 400 hours and climbing; I am very pleased it. There was definitely a steep learning curve (as in, the first one flying with this engine...), but working with UL and Ray Laurence at Kaolin Aviation, it's running very well now. I have the setup dialed in now and Ray has a FWF package. There are a bunch of RV-12's with UL engines flying now; the first one's were the 260 engine, but my recommendation is for the 350, mostly because it performs very well in the airframe and isn't a whole lot more than the 260; it was something like $2K difference in 2016 when I bought my engine.
The engine, much like most air cooled engines, uses oil as a major engine cooling function in addition to lubrication, so getting the oil cooler air flow correct is important. I used piccolo's before and after the oil cooler to provide differential pressure data as I made changes in the cooler, cowl and shrouding. I have that worked out now and operate within the oil temperature band without issue.
I want to clarify that the alternator is a permanent magnet alternator, meaning it operates on the same electrical principle as a standard magneto; if the engine is spinning, the alternator is making power. If the battery fails, the alternator keeps running. In fact, if you wire your battery such that you can isolate the battery from your electrical system, it'll keep purring right along. I can attest to this as I verify that operation routinely. Having completely disassembled and inspected the alternator (all two major parts
) - I can say it's well made, (by Cycle Electric in Ohio, no less.)
I bought my engine used and retrofitted to the 50A alternator and highly recommend getting it as an option if you're buying new. It also has a rectifying regulator instead of the shunt type that the 30A comes with.