My feel/understanding is that chamber design is something that gets more difficult as it increases in size (obviously), but also by combining multiple smaller chambers you only suffer minimal downsides from a reliability perspective. After all, it's not like multiple smaller chambers result in heaps of extra moving parts, so a lot of the risk of duplicating components is gone.

You're right, though, the R-7 is a fantastic achievement not only for the chambers, but also considering how many engines and pumps it has. All those spinning, moving parts are another point of failure and they're all necessary for success.