While it's undoubtedly true that if things are going well, people are happy, there's lots of statistics plotting the number of cofounders vs the chance of success. Each additional cofounder beyond 2 drops your chance of success materially. 2 > 1 > 3 >> 4 >>>> 5+.

It's exceedingly rare for things to go perfectly and as with any other relationship it's how you handle the situation that matters. More people mean more risk that you don't navigate successfully. But of course being the only one means literally everything's on your shoulders and you don't get anyone to bounce ideas off of.

Yes, things going badly creates friction, the question is whether your cofounder relationship helps or hinders, and it sounds like in most cases it hinders. So both can be true.