Hearing Fidelity blocks various types of apparently risk bonds, ie all Puerto Rico, and the like, among others. Probably not a great place for bond investors, and they’re bad on some types of preferred stocks as well (can’t trade online, but can call in orders if you care enough). Also heard some bad things about their FX conversion costs on foreign dividends.
For Merrill, it seems they recently imposed a 100 max contract limit across all uncovered options, regardless of exposure or number of stocks or whatever. You supposedly can get this increased with some higher level review process, but it’s not fast.