I play ele and resto offspec and I mostly Raid (and weekly M+ on top), I chose Necrolord.
The shield is somewhat lackluster (I don't foresee too many chances to use it in raid encounters) but Primordial Wave is awesome. It's also the one ability that will scale really well with legendaries like Skybreaker's Fiery Demise and Primal Tide Core, which are alrealy really powerful on their own, but Wave makes setup much easier and less painful (#removeflameshockCD).
Also lavabursting into 4 targets makes us even more powerful in funnel/spread AOE type of encounters and just feels fun.
For Resto, having another riptide lets you set up the Big Brazilian Necrowave, which is going to be especially useful in M+ and, again, just feels really good.
I've tested the other Covenants, Kyrian totem felt frustrating to use if the tank kites, Chain Harvest - really cool in dungeons, but lacklaster for single target, Night Fae goes the other way around (but the Soulshape is OP, just for that NF would be my second choice).
Really happy with my choice, even though I don't care much about Maldraxxus lore.