I think co-founders are great but only when you get them RIGHT. If you didn't get them RIGHT, then having co-founder/s will just be full of pain and frictions.
I personally had multiple ventures where I had co-founders over the years let's say 4 times. Not every time I was able to get this right, due to lack of experince probably.
When this was right it really felt amazing to interact and work with the team, things were moving super fast with very low friction.
The most import thing, I think comes down to ALIGNMENT between the co-founding team.
What type of alignments, you might ask?
I can think the following list is quite important to be aligned about ..
1- Vision (near and far).
2- Approach: the approach you take to get to where you want to go, including methodologies of work.
3- Seeking Funding vs Bootstrapping (and at what stages).
4- What metrics to watch (vanity vs true, etc)
5- Responsibilities and expectations form each one.
6- Breakup (how would you handle exits if one of you just decided to quit)
These are just based on my experience with co-founders so far ..