I love SuperTuxKart. It's the first game I install on any new computer, and I've beaten the story mode several times. I usually focus on the racing modes, but I've tried Soccer as well.

SuperTuxKart is not an alternative to Rocket League! If you go into it expecting anything Rocket League has -- polished realistic-looking graphics, physics that let you jump and tilt and drive up the walls of the stadium, a thriving online play community, a matchmaking system -- you're going to be disappointed.

Your criticism is correct. It is similar in the gaming principle but way way behind in user friendliness and polishing. That said, as far as I know, this is the best alternative that we have right now on Linux. Having played it with kids (who never played Rocket League before) I can say that it can be a fun activity.

This is the "just use GIMP" of games.

While true in theory, the gameplay vastly differs to say the least.