I have three basic problems:

1) hoisting the spinnaker quickly and efficiently
2) steering while hoisting (and dousing)
3) dousing the spin

I've seen a lot of Uni sailors do those things well, but none of them are as quick and efficient as a sloop. Surely there is better way to steer than with a "butt cleat".

Getting back aboard is also more of a problem for Unis than sloops. I think that prompts a lot of Uni sailor to be more conservative, especially on the down wind.