VDH alt interested in trying havoc out. Momentum seems really interesting to me.
On my main I ran a key the other day and saw a momentum DH pull 7k overall.
Is this build standardly viable? And if so what covenant is optimal and how does it fare on ST?
Thanks in advance 
It’s not, not viable.
It’s just not preferred because it can be impractical to use. The talent forces you to use your mobility tools for damage instead of… mobility… Also without demonic you lose a regular source of self healing
In general, night fae is the best covenant for havoc general use. Venthyr is good for single target. Kyrian good for M+. Even necrolord isn’t horrible anymore if you like it.
Viable? Meh. If you like it, you can make it work, but probably not particularly well.
Why?
In a word: Movement.
You’re sacrificing movement for momentum build. Movement is the name of the game in Shadowlands. Frankly, it was in BfA and most of Legion, which is why the momentum build isn’t taken very often. And a 6 second damage window really isn’t that much. The haste granted from a Cycle of Hatred/Demonic build is far more reliable. There are some momentum build dhs in the top of the ranks, but demo is more common for a reason.
But that brings up another point…
Unless you’re pushing mythic raids or 10+ keys, it’s probably not going to matter much, so play which build you prefer.
My personal opinion is that I much much much prefer demo build. It has better survivability, and I like the legendary I use (which I’m also thinking is going to be a bit better come 9.1 with the buff to chaos strike/annihilation…) While it suffers a little in output, I find it to be a mechanically more interesting and fun build to play, and it still does good enough dps that I am actually fairly competitive in what I run.
Most of the top havocs on raider io run momentum. It’s harder, since you’re sacrificing movement, but it obviously can perform very well in mythic plus.
Most of the top parsers in wowlogs for M+ run demo, and pretty much all of the top raiders use demo. 
Momentum is absolutely viable in keys, most Havoc DH are Nightfae.
It’s a fun build, I play it 100% of the time and I keep up with most of my guild’s dps in mythic raid just fine, I hardly do any m+. I play night fae cov.
not viable, you wont be able to heal, you will get out of the fight
etc… not really worthy.
Seems like it might synergize pretty well with the new nightfae lego for m+ in 9.1.
aura > UBC > eyebeam > meta > eyebeam > hunt (with new aoe buff from conduit) > UBC
all over like 10 seconds and all benefiting from momentum. I doubt it’ll be the meta but it seems like that combo could be pretty busted under the right circumstances.
…so yes it is viable…but it has it’s issues…Unless you’re really good with distances and positioning. Don’t use it.
Hey man, my recommendation is to watch Jedith on YouTube; he has a LOT of videos on Havoc DH stuff and is thorough, intelligent, and explains EVERYTHING. He touches upon momentum at one pt, and said it’s not as viable as it was. Anyway, unless you’re going for super-min/maxing, do what you want. It can be made viable, isn’t terrible by any means so, if you enjoy it, go for it!!
It is viable if played right.
Pretty much all the builds havoc can use now are solid. I’d say use what fits your play style and you will do fine.
No is not since you can’t heal yourself at all, easy way to die in pvp.
M+ no one move the packs that much to fel rush everywhere so won’t be up most of the time.