Imagine if there were dozens of challenges every single day that you could check and decide which ones you want to do, that would make gambling so much better for many people that enjoys that kind of stuff.
Basically it is fine to let all players start a challenge but there should be a requirement of minimum reward to avoid dozens of unrealistic challenge.
I mean by unrealistic challenge is challenge with small reward but hard task, for example a challenge with $5 reward but the task is to hit 500,000x in limbo or mine.
I'm not really a fan of challenge, I do not check the challenge tab frequently but I did try some previously.
Indeed, that's really nice suggestions there are an unrealistic challenge in terms of rewards, limiting those and sorting will give players delight to join and participate, it's good to add some spice while playing, when you are trying to win and there's additional task that you also aiming to achieved, that's encouraging.
But, in most cases like that example you mentioned, it's not interesting though there's no forcing to anyone to one but when seeing that kind of challenged, personally, it's annoying..