I still highly recommend Mullvad, they are generally an awesome company and havent had many issues with them beyond a few random sites blocking them which is to be expected.
There’s no technical difference, but there are differences in practice.
First of all, in order to transfer any content, one of the participants need an open port. In case of public trackers, there are usually a lot of users, so someone will probably have a port open that you can connect to. Also, many don’t even realize that they have open ports, since routers can open them on demand by uPnP. For private trackers, since the userpool is much smaller, these matter more.
Also, for public trackers, you don’t really HAVE TO seed and maintain a good ratio. It’s the right thing to do, but is by no means a necessity. For private trackers, on the other hand, your account will be suspended in most cases if your ratio goes too low.