It’s not a simple answer RE: Brutal Momentum.
The attack pattern you can find on wartide here.
What happens is the pattern goes L1 → L2 → L3 → L2 → L3 → L2 → 3 and repeat. After the first L1 you won’t see it again. This is important to know because when you look at L3 you’ll see it has no damage target cap; it can hit as many targets as you can cleave.
Where the complications arise is that L3 has a cleave rating of only 1.2. I linked earlier my bug report about this since as a vanguard attack it should be cleaving more than that. That post is linked here.
In practical sense what this means is that your attack chain of light attacks is L2 → L3, and L3 hits 1-2 enemies at best. This is terrible for horde management.
If you have Brutal Momentum the cleave on L3 is unlimited, and that means you can hit (and damage) as many targets as your weapon swipes through. L2 is unaffected as it will only damage 3 regardless.
While this might initially seem great the problem compounds because it’s not easy to hit heads with a Shovel since it prioritises body shots instead of heads. H1 will reliably do so, however. In the end BM means 50% of your attack chain has unlimited damage targets (L3).
The last point to make is that the damage drop off per target for Shovel is savage. Your second and third target take tremendously less damage than the first, so even if the target cap is increased I don’t think this will “save” the Shovels horde clear. It needs a little more than just this singular buff to help it out.