I'd say watching an STV demo of a close match is probably the best way to improve other than just playing the game. Watching your own POV demos can give you some insight into mechanical things like 'oh my movement was shit there i could have easily dodged that rocket and lived' but not being able to see your teammate's or opponent's health, numbers or ubercharge (unless you're medic and even still you can't be sure of the enemy uber) means there is a lot of information missing.
It can be helpful to get somebody better than you to mentor you and point out mistakes and give tips/suggestions of how to change your playstyle for the better, bud imo a lot of newer players have a mindset that relies to heavily on some invite player spoon feeding them info and holding their hand all the way through. At the end of the day the only person who was control over how you play and improve is your self, so at a certain point you have to start recognizing your mistakes and taking responsibility for your play.
Another thing is that I feel like doing a demo review with your team can be a lot more telling than simply doing it alone or with a mentor because you get to gain insight into what is going through your teammate's heads and their decision making and are able to, as a team, decide what you did wrong and which decisions were right but didn't pan out because of people missing shots or lack of coordination, etc. If nobody on your team is at the level that they can confidently recognize mistakes and errors in your play overall as a team then it's probably useful to get input from somebody with more experience. A lot of the time though, just knowing what your flank (or your combo if you play flank) was doing in a given push can help you figure out where the disconnects and miscommunications are in the way your team plays. Also, no matter how revealing your personal demo reviews may be, if you aren't talking together as a team, odds are the more general flaws in your teamwork will remain even if you personally feel like you're playing better or more optimally.
That said though, it can definitely be helpful when starting out or to look more specifically at the mechanics of the game to do personal demo reviews, I just find it more helpful to assess the overall team play and decision making (probably because I main medic and there isn't much to the mechanics other than dodging/hitting arrows and the occasional surf whereas on scout or other DM classes it's a lot more important and complex)