From what I've been reading, "free" certificates are trouble. I ended up buying a cheap $9/yr cert from Namecheap (who resells for Comodo) for my website, and installation went pretty smoothly.

As far as compatibility, there are a few places in the code base where "http" is hard-coded, notably logout.php. This can cause browsers to (rightfully) complain about "mixed content", so we should fix that to use the appropriate protocol for https versions of websites that implement UserFrosting.