Mastery and Multistrike are about equal, a good balance of both is recommended.
WE used to go all out MS because it was a little bit OP but then got nerfed by 30%. So not OP anymore.
It is still very useful, just not worth going 100% MS 0% mastery anymore.
Cenarius put it well. Only reason to go 100% MS is for damage and tbh MS isn't even the best dps stat generally. Most items that have MS have the better dps stats so w/e.
I know right now im sitting at 2650ms and 950mas unbuffed. It feels 100% safe even on Archimonde(he actually melees less hard on mythic than heroic looked at logs lol). Anzus is enough and then a dps trinket is 100% fine, but you can always use WuE/Class trinket in the 2nd slot if you REALLY need to live or a BRF trinket.