Bro The boss Rush is pretty relaxed with only 5 fights . So the rewards are actually pretty good considering it only takes like 10 minutes at best to do the rush
Kabam instead puts Boss rush rewards in those random 8000 units sale.
20 fights since you have to do it 4 times to get okay rewards. It may technically have better rewards than any other boss rush, but that's because boss rush rewards have always been relevant for account progression. These rewards are not, therefore it is very poorly rewarded for the difficulty and effort. And it is especially bad being disguised as a side quest.