Myes, if you play a bunch of games against Rainbow with CCYB, you will lose most of them.
+30-card Rolhope decks on the other hand can actually win all of them provided even mediocre draws. How? Deckout. Winning by stalling enough to Fractal Archangels (the fastest option) past the Miracles is your second best bet, but won't happen very often. Since deckout is the easiest method, many still opt to skip her as it takes ages. Also, most run 30-card rolhope decks, which makes the decking much harder. You have to get lucky with how early Rainbow gets her Hourglasses out.