It gets boring raiding for sure as disc–especially if you play with the same group all the time. You learn the boss fight and pattern, then rinse and repeat–and that gets dull. Because disc has mana issues you have to sit on your hands at times or you go oom, too, so people say disc can get very dull because of that. Basically, raiding, you end up pushing your buttons for certain things, and then you sit on your hands, then go again, or just smite between the dances. Basically, you learn when to counter what, and then do it and then do your thing and then sit on your hands. Mind you, in my case, I have pugged a lot and when pugging found you can experiment a lot more vs with a guild that locks you into doing exactly this and exactly that. Versus holy though, it is a lot more limited. With holy you have a lot more room to do things this or that way, but you often will not pump the damage you can as disc. So there is a trade off.
Oh, another thing when comparing disc to holy is that holy is the most resilient of all healers. That is, they die the least of all healers. Disc is actually the opposite–in that they die the most of all healers. As for the whys and wherefores, I am not completely sure, but think it is for a few reasons–like mobility and having a different spell set (holy has potent spells for saving anyone including yourself), but wont get into that unless you want my detailed thoughts on that.
In the end, all healers can be dull though–once you learn the fight pattern. With disc, it just happens sooner. I will say this though, if you pug it is a lot less boring. Each group does things differently, so that can add an element of challenge.
As for M+? I find disc to be super fun in M+ and the most fun of all healers there. And again, if you pug M+ it can be incredibly fun because you never know how things are going to go, and you get this added element of challenge because you have to adapt to the different groups. While some find this a problem, you sound like you are like me and like that.
That said, unfortunately, disc does not pump the damage of the meta healers, h pala and r shammy. And there is something of a stigma when it comes to inviting priests. Hence, if you pug, while it can be more fun than meta, you tend to get less and less fast invites the higher the key. Also, I think holy is terrible in M+. It was fun in BFA when you could pump healing with Prayer of Healing and dps a lot between casts while Echo of Light was ticking. Now you are simply a spot healer with Flash Concentration–which I personally can’t stomach because of how dull it gets.