Personally one of the biggest red flags for me is when a developer doesn't use libraries and writes everything themselves. That opensource project with many contributors, that has been around for years is probably going to be better than what you can write in couple passes. There are obviously times when you need to do it yourself or the library truly is badly written but those times are rare.

This gets a bit weird in the JS optimization world because it is true that each library and parts of the library you don't use are going to be extra useless bytes the user has to download. With tree shaking removing the parts of the library you don't use I think this becomes less of an issue.