Remote co-founders, yay or nay?

I would say no.

I would advise never to start with a distributed team. Sure we have the tools & technologies to shorten the distance & the communication-gap, but nothing beats the in-person talk where people can tap on your shoulder to discuss ideas, disagree with you & informally communicate with you. Once you enable the team to take decisions on their own without your active involvement, that would be an appropriate time to go distributed. My learning is that the team has to be decentralised (in decision-making) before it's distributed. Otherwise, founders could end up becoming the bottlenecks in startup's growth.

Please take my opinions with a pinch of salt as I've never been a founder myself, but I was part of a geographically distributed founding-team member of a startup, which got acquired ~3 years ago.

I would say nay, for a number of reasons.

Let's take the somewhat sunshine scenario first. You make something that actually succeeds. You start earning a lot of money. Has each of you contributed as much to the outcome as the other. And will both of you say so about a person they have only met online. Prisoners dilemma. Or at least could be if one decides to screw the other. And I think every piece of research ever done about behavior online points to the fact that people are less empathetic towards each other online.

Second point: I have started to believe that the most important factor in any endeavor, whether it being starting companies or otherwise, is to do it with people you really enjoy spending time with. Whether you succeed or not you have to make the journey, that journey might as well be with someone you really like.

You can of course find someone online that you end up really liking, but I would advise against a "fully remote" relationship.

I would says 'depends'. No if you dont know someone very well. If 2 people know each other well e.g. long-time one line relationship or lived close before and now happen to be in different cities that would be fine.

I have done it with side a project. We worked together (remotely) on company jobs for few years communicating almost daily. Id say no problem if you know the person well and the relationship gels.

Fwiw SendGrid started partially remote, but the folks already had a food working relationship. Two were colacated in Colorado for techstars and one was remote. However, the two went back to colocation with the one as soon as the incubator let them

I think anything is possible. There are an infinite number of options for starting a company, and a remote co-founder (or even a few) is not a problem, in case the cultural, ethical, moral code coincides.

So look for people who are close in spirit and vibe.

Big Nay. There are enough challenges starting a company already. Remote will make it 2 times harder.

Fwiw SendGrid started partially remote, but the folks already had a food working relationship. Two were colacated in Colorado for techstars and one was remote.

It likely will or has worked at some point for someone somewhere. Why not you?