few people touching the code is easy mode IMO. much easier to not introduce bugs if you've designed half the system and are deeply familiar with its various idiosyncrasies. good architecture is not a must here, although fewer people obviously also make it easier to stay consistent and make larger refactors at once without stepping on anyones toes.

sometimes that bug free thing that just works and was written all by one guy is just a real pig behind the scenes, but hey, that guy always knew what not to do, no matter how messy the APIs.