Who said they are unbeatable? But the reality is they have proven they can win with just middling QB play because they are built to not make the QB the focal point of their team. They play elite level defense, they can stop the run, they can rush the passer, the don't make mistakes. Offensively, they have good OL play, Shanahan is a great run design/play caller, and they typically can control the flow of games. We saw it up close last year.
And yeah, they haven't won a SB since the 94 season but since then they have played in 6 NFC Title games and 3 SBs. They've been a crap ton more successul than we've been, that's for sure.