UTNL If someone impersonates you, do their stats get logged against your username? Because if so, that definitely sucks if someone can come along and destroy your stats just by using your name.
Generally, I think both stats systems (the website stats and the team balance stats) are safe from this because they should be tied to an IP address and name BUT I have noticed a few issues that makes me think there could be some possible loopholes to this. Both of these systems I'm planning to do work in and I will definitely sort these issues out if they exist.
Since you mentioned that you do some coding before I will mention that both systems have their source code available if you wanted to check on this yourself and get a more clear answer to your question.
The UTStats that I'm using on the website you can check out here: (I've made some changes to this already that makes it differ from the repo but nothing that affects what you are asking about yet.)
The balancer that tracks stats in game and pops up the leaderboard when you enter the server is available here:
The .jpp file is the one you want to check out in that zip file. Since there are lots of comments and meaningful variable names it is pretty easy to follow even if you don't know anything about UT/unrealscript.