There are a couple of downsides to the UNA rig. One is that there is a lot of windage forward, and that causes it to sail at anchor (lot of sloops do that too, but the Nonsuch maybe a bit worse). Some owners handle this by setting a riding sail or triangle which reportedly works well - I have never tried it. The other downside is with the single sail, you cannot control the boat when standing still: no heaving to, etc. Sailing off an anchor can be a bit tricky for example. It is possible to stop by simply letting the sail out until it is luffing with the wind abeam (no shrouds so no limit to easing the boom). The boat will stop and be quite stable in that mode, for reefing or what have you, however you are beam to the seas and the sail is luffing. I only did that temporarily to reef. I can think of no other drawbacks peculiar to the rig.

These two issues are the main reasons I have a mizzen on Anomaly, it has been a complete solution to those and some other problems as well.