I agree these should be off topic
In addition to thinking that a winning criterion should be fair, I also prefer to see challenges where the submissions are all solving the same task and can be compared like for like.
I agree with avoiding bonuses in code golf, because it creates several parallel competitions which are jumbled into one. In the same way I think giving a different task (or different advantages) to different people means that the list of submissions cannot be meaningfully compared.
A challenge should not just be fun, but meaningfully competitive. That's the specific type of fun that this site caters for. A challenge here should provide a level playing field and a straightforward way to assess which of two submissions is winning.
Tackling a PPCG challenge can provide competition with others, and also competition with oneself, both of which can be rewarding. Giving different challenges to different people still allows competition with oneself, but removes the significance of competing with others, losing a large part of the competition.
I also think challenges should be inclusive. To be told "you can't play", even if only due to an accident of username/avatar that renders your possibility of competing negligible, is frustrating. Rather than simply discourage this situation, I believe it is best that this be declared off topic, so that all of our challenges will be available equally to all.