Civil war. Social upheavals. Gridlock. Oligarchs.

Didn't the U.S. also have all those problems, despite having a president?

There was an assertion made in an ancestor comment that a committee without a leader can't function, but that doesn't seem proven by those problems in Roman history under their Senate, since the U.S. with a president had the same kinds of problems.

I assume other factors are at play, oligarchs for example. And I question the downside risk of having a strong presidency, cult of personality for example.

I agree that the term "did just fine" is subjective.