The concentration thing is a big deal for stacking. As it is now, you can stack HM with Hex from Magic Initiate or something like the Smite type Ranger spells.
I'd like to see the math on how it worked with the different spells stacked vs. a fighter or barbarian's damage output of similar level.
A quick look has: 1d8 Bow +1d6 HM + 3-4 Ability = 11-12 per hit at 1st level for a couple of HM hours potentially per day
Barbarian has 2d6/1d12 + 3-4 Ability + 2 Rage = 11.5-13 per hit for 2 combats a day, 9.5-11 the rest
Fighter (BM) could have similar: 2d6/1d12 + 3-4 Ability +1d8 for 4 hits per Short Rest = 14-15.5. 9.5-11 the rest
Or Fighter (BM) Duelist could have 1d8 + 3-4 + 2 + 1d8 for 4 hits = 14-15, 9.5-11 the rest
So even without stacking any other spells on there, the Ranger at 1st level is going to dish out more damage on average than a fighter or barbarian for most of your combats in a day. Doing that without Concentration is a pretty big deal.