From an accountability standpoint, if each person logs in to the root account directly with a distinct private/public key, you can still have full individual accountability.

I'd think the complexity side of the sudo strategy is self-evident, so perhaps I'm not understanding the part that needs explaining.

For accountability the openssh server already logs the key fingerprint.

The main security implications of sudo are a false sense of security and the risks related to having a password on your account and typing it in all the time (how often have you spilled it into a bash history?).

Sudo can't provide a dependable audit trail because it is trivially circumvented ('sudo bash'). It doesn't protect you from local-to-root exploits either.

It doesn't even protect you from yourself but rather makes your critical commands more complicated and error prone (shell globbing/escaping, pipes and redirects, etc.).