We base our gear advice on simulations that we do with the healing simulator that we created in legion.
The first thing I tell anyone asking about healer gear advice is that it’s not like DPS. There are going to be multiple ways to gear that can all be successful. There isn’t really a way to definitively say which set is “better” than another if both sets can be used to successfully keep your raid alive.
What we do is set up our simulator to keep a simulated raid alive using logic that is similar to how players would play. Like, use prayer of healing if there are 5 people in range of it who have less than 75% of their health. Cast HW:Serenity on someone if they are low health. Favor the lowest health targets for heals, etc.
Based on that simulation, we find sets of gear good at keeping players alive. Then, we sort all those sets of gear by picking the ones that do the most healing per second as “best” - even though that can be considered a somewhat arbitrary metric.
That method rarely ends up favoring mastery as the best stat for holy priests, since it is mathematically not as strong healing per second as crit and versatility (assuming you get some of each). And, you can keep a raid alive without stacking mastery.
I think other people favor mastery because they just like it. And, if you are using simpler models of the game to calculate healing per second, it often looks more attractive than it is if you do a full-on simulation of a raid environment. We are the only people using full healing simulations that track an entire raid environment to find the best gear.