I've been playing video games since NES, and I've played them all, console, PC, mobile, I think I can safely say, I've experienced all the platform.
Grinding can be fun, if there's a small way point to look forward to.
In the case of raid, this was true from 2 years ago up to 9 month-ish ago.
Back then the progression mission is that small steps to look forward to while grinding. Not so much today, once tou hit that Bronze 3 mission today, the small steps suddenly becomes a ravine that you need to jump over while you've just learned how to crawl.
It was fun for those who have played for over 1 year, because the small steps is still there, but for new players, the grind becomes unbearable because their next reward becomes so far away it's unattainable.
I think the best way to illustrate for veteran Raid players is for you to try other game, like this one
Dead Ahead: Zombie Warfare.
It's a fun game with solid gameplay, BUT like Raid TODAY, there's a point where progress comes to a screeching halt, where the grind becomes unbearable because there's nothing to look forward to. What is there is daily dose of doing the same thing over and over without any sign of progress, your heroes upgrade becomes meaningless because it can't overcome that next step. In Zombie Warfare case it's around level 6, for Raid today is around Bronze 3 or Silver 1 mission.
Grind can be fun... But without anything to look forward to, it becomes time to ditch a mobile game