Holy’s niche is that it’s a jack-of-all trades, master of none. The spec has the most overall healing spells out of any other healing classes which allows it to respond and adapt to any kind of raid encounter. As a raid healer it can put out fairly solid throughput, has two raid-wide AoE CDs (all other classes only have 1), and can restore all the healers mana (twice in a fight if you time it at the start & end). You can literally play like a druid throwing HoTs on everyone or spam nothing but your AOE heals. So if you don’t like being stuck in one particular role then it can be fun.
In M+ holy is able to use its toolkit to pump through affixes like bursting, necrotic, etc. that give other healers a hard-time (especially Disc) and can do fairly decent dps (priests are on the bottom end of the healer DPS totem pole) as long as you properly weave your damage spells inbetween heals.
Utility wise holy also has a spammable tank CD with Guardian Spirit resetting to a 1 minute cooldown when talented. It doesn’t reduce damage taken, but allows the tanks to self-heal themselves for 65% more (tanks really do like 80% of their own healing), and also your own heals do 65% more. This gives you that kind of comfort zone where you can safely throw it around and when it does trigger: you just saved somebody from dying.
Most of the other utility is shared with other priests in general with Mass Dispel and Leap of Faith. Mass of Dispel will become more useful come pre-patch because you can now remove affixes with it like bursting. We can also shackle undead, but that’s not used at all in M+ (ccs pretty much aren’t used ever).
You also have Psychic Scream and Shining Force to interrupt adds, but requires aggressive gameplay, and being aware of your surroundings. This is especially useful in some dungeons where there’s mobs that do AoE damage or their spells will hit the tank really hard if cast. It also takes a bit of experience to actually learn what you need to help interrupt.
Healing wise Holy Priests can really power through anything that’s thrown at it in M+ because you have a big arsenal at your disposal. You can heal two people simultaneously with the Trail of Light feat with Flash Heal/Heal, dump renews, or pump out a few aoe heals and top the group off.
You’ll start seeing a decline in popularity around the +24/25 keystone which really is the extreme end of the rankings (its like being in the 3k arena rankings). It’s not because the spec is bad, but priests generally aren’t desired in the groups of the top M+ players (even Shadow). High 22+ keystones are usually run by groups instead of being pugged because everyone has to be on discord to coordinate.
I had the #7 priest on the EU rankings tell me this is the case. Below that… like the casual 1-15 or 1-20 range you’ll be able to find groups without problems.