That's a bit disappointing with regards to 4.21 REX and being less compatible with some games than 3.55 CFW. Just the other day I finally moved up from 3.41 OFW + dongle to 3.55 KMEAW with a view to going REX during the weekend. KMEAW opened up a fair number of games that simply wouldn't get going under 3.41 + JB regardless of Multiman options and numerous eboot fixes so I'm happy for now. I'm just slightly kicking myself for not having retired the dongle sooner!
Anyway, will you be downgrading back to 3.55?