I just posted a feature suggestion related to improved handling of honorifics and surnames ("Dr. Smith" not being split across two pages of CC text), and this goes hand-in-hand with my CC issues.
I haven't found a reliable way to add to a user dictionary for spell-checking, or a good find/replace option for editing captions once they've been generated from a transcript in Premiere. I'm working at a school district with many ethnic surnames and many acronyms spoken during Board of Education meetings. It would be very helpful to add our Board members' names and our many District-used acronyms, campus names, course names, etc. to Premiere so captions are more accurately generated. I think I tried once or twice to edit the spellcheck library but none of what I've added ever seemed to do anything at all. What I wind up doing is manually playing the Board meeting video all the way through (often three hours' worth) and manually tweaking every screen of captions, or exporting the caption file and bringing it into BBEdit to globally replace misspelled names or acronyms. The problem with the latter approach is that the misspelled names are different every time, so global find/replace work is iffy, at best. More accurate auto-transcript and CC text would speed things up considerably, so a reliable user dictionary is a must.