I run a very small XenForo forum on MDD. It has always run without issue. When I had 15-20 concurrent users the board ran just fine. Once your north of ~40-50 concurrent users you need to start looking at higher end shared hosting (Premium Hosting) or a VPS.
Generally, speaking all my sites hosted here run without issue. One time a client of mine failed to update his WP theme. He was hacked and his account was sending spam. Support was great about working with me to bring the site back online after I cleaned it up.
Do you run all your sites off of a single shared hosting account here? What kind of "visitors/page views/bandwidth" do you use monthly?