Default project handling doesn't work at all. First of all, the 'Remember project per' options don't really make much sense. E.g., if you use it with Trello, you'd most definitely want to do it by board. (Which should be similar for all PM software.) Per url means per card. But let's say the sites are too different to make it work this way.

Now if I say 'per service' (i.e. use the same default for Trello), it will just pick a random project (always the same, presumably the first project I've ever created on Toggl). No way to set it, no way to change it anywhere. So you can either use the per-url setting (but if you screw up, you'll never be able to change it for that URL or, at least, I have no idea how to do it) or the global default. Both very inconvenient when you work with multiple projects.