I enabled the HTML 2 years ago: just never told my teachers about this- I don't want them attempting to add HTML to places such as activity titles.
1 notable consequence if you do add HTML to activity titles - I use span tags to color my assignment names in red in my orientation course. This causes the assignment names to not appear in the activity completion report. But the titles do appear in red in the grade book/userreport, so it's a trade off I'm okay with so far
See 1.a/2.a/4.a (again, this is if you use HTML in the titles).
If I recall anything else I'll post.