As a watch enthusiast, I've wanted to build my own watch in the past. However, as I got deeper into watches, I realized I could never really make the watch my own creation unless I developed and manufactured the movement myself. Otherwise, I'll be dressing up a generic ETA movement [0] with a personal dial, hands, bezel, etc. This is what most of these DIY watches are. With that said, ETA movements are some of the most solid movements out there with decades of support. They're also capabale of COSC [1] chronometer timekeeping if adjusted properly. For me, the DIY magic dies with the movement because I'm limited to the dimensions of the ETA movement. It's the heart of the watch.
[0]: https://en.wikipedia.org/wiki/List_of_ETA_Movements
[1]: https://en.wikipedia.org/wiki/COSC