When Dropbox came out, many people LOL'd (especially here on this site where most people could build Dropbox in a weekend) because it isn't that complicated. It was just pretty and targeting the less-technical.
But now here we are, on the same site where people used to be able to build this in a weekend, asking how to find an alternative because (presumably) nobody can build this in a weekend.
The original Dropbox could be a viable product to disrupt today's Dropbox for many of the same reasons Dropbox originally achieved success.
JQuery was still the way we did things during those times.
So, yeah, I guess back then, things were relatively straightforward to build something like this. Today, there are many OS’s, file systems, ipv6, shitty wifi, and a whole host of problems that didn’t really exist back then. Heck, back then, most people didn’t even have laptops so you didn’t even need to worry about your programs going to sleep in the middle of processing.
99.99% of the time only a single file changes at a time, which is the use case here.
But you’re right that there are many such error cases to handle if you want a real iCloud / Dropbox alternative.
Installing on a desktop, laptop, and mobile device would bring 3 clients into the mix right away.
So the user’s first use is likely to be one of the more complex operations, and the one they absolutely needs to work, as a failure here will lead to the user looking elsewhere for a solution.
You are almost guaranteed that the first sync results in zero conflicts. It's just a mass-upload. Even if you bring other clients in, there is still a very low chance and "last-one-wins" is a pretty decent policy for who gets the original filename and then other gets a " (copy)" suffix or something. Doesn't really matter as long as it's documented and intuitive.