There's been regular investment in MUC throughout OpenFire’s lifetime, but we have no idea if it'll continue to work, even though this is a core feature that I feel almost all installations would consider "mission critical".
Unit test coverage is 4%
The XEP contains 77 sections, and is covered by only 4 integration tests from Smack.
Look for opportunities to add unit test coverage
Contribute integration tests to Smack (or sibling repository) to give confidence that it’ll continue to work in production
A few proposed here: