Right now the biggest barrier for me is the interface. Too many instances favor this very spread out, low information density, optimized for mobile (but not really) look, and group information in ways that are irksome. A certain amount can be fixed by user styles, but some functionality can't, like on kbin when I click on reply notifications and am not actually taken to the context of my reply because the comments have split across multiple pages even though I've set my preferences to infinite scroll. If I'm on another instance, clicking context gives me... well, not the context of my comments, just the damn comment and a series of "load parent comment" links to click. It's, well, irksome.
Most of my gripes aren't insurmountable, but that doesn't change the fact that right now every instance is a little janky to use.