The Rune Knight might be a little fussy and need a few balance tweaks, but it doesn't have a patch on the broken subclasses from Unearthed Arcana of yesteryear like the loremaster. The subclasses on UA in the past eight or nine months or so have been much better, probably because they are generally not as ambitious as the Mystic was back in the day.
Of course, that does not mean the Rune Knight is fine tuned balance wise, UA subclasses tend to be front loaded, as balancing for multi-classing is not really taken into account, and some of the mechanics need to be smoothed out. I would probably restrict multi-classing with the RK, especially "cherry-picking," but I don't see it as too out of line or as "out there" as the original mystic was. That said, I haven't seen it played, so ymmv.
Of course, that does not mean the Rune Knight is fine tuned balance wise, UA subclasses tend to be front loaded, as balancing for multi-classing is not really taken into account, and some of the mechanics need to be smoothed out. I would probably restrict multi-classing with the RK, especially "cherry-picking," but I don't see it as too out of line or as "out there" as the original mystic was. That said, I haven't seen it played, so ymmv.