I love the new hero version of Elfhelm. He is a subtle hero that will never be the key component in a broken power lineup, but encourages and rewards fun and unique deckbuilds, especially with more players. Here are a few observations from playing with him so far.
1) It is important to realize the Elfhelm actually buffs two things: mounts and color fixing. Theoretically if each of your heroes is 3 color with a mount Elfhelm can provide +9 in total stat buffs just solo. Amarthiul and Aragorn (with Celebrian Stone or STWB) are great to combo with Elfhelm because they color fix so easily. Other candidates need songs, which is easy enough, but it's better when you already want to color fix anyway.
2) Elfhelm provides the only global defense buff in the game. Defense buffs in general are very rare. This makes him excellent in siege quest situations and it makes Erkenbrand and Amarthiul (again) great candidates for his buff. Unfortunately the way that defense works he will rarely benefit from his own buff because it is to risky to use him to block unless you are using him against small enemies. For this reason he himself needs color fixing as well as a mount to get any use from his own ability, which feels like more work than it is worth. The biggest benefit to Elfhelm is obviously the armored destrier which every defender naturally wants anyway, especially in leadership.
3) Legolas is significantly better with 4 attack and you normally already want to give him Arod or Rohan Warhorse, making him an obvious combo choice even without color fixing.
4) Glorfindel has a mount called Asfalof, which is pretty good. If you can give ol' Glorfindel LOV he can quest for 4 and be ready for attack. With song of battle, he might just be one of the best heroes in the game. Perhaps Elfhelm can finally unlock his potential.
5) It feels so wrong putting mounts on Dwarves, but if you can stomach the travesty Oin turns out to be pretty good with Elfhelm. It probably won't happen much because anybody running Oin will be using a Dwarf build.
As the cardpool grows Elfhelm will become better and better. More mounts, more color fixing options, and more ready effects to utilize all the state buffs will make Elfhelm a hero like Caldara, a hero that will shine brighter in the cycles to come.