The best 1v1 halo was Reach zbns due to the discrepancy in ease use not being so large between utility and pick ups. The pistol in H5 is great but it's power and ease of use compared to the rest of the sandbox is trash. Not to mention sound whoring being very relevant in H5. I still make 1v1's in H5 put they are typically pistol driven or have some weird weapon set because in Halo 5 you really have to make a choice. Do you want your map to be about weapon control or gunskill as the imbalance between the pistol and the rest of the sandbox doesn't give you very many options in between that.
I think another issue with 1v1's in Halo in general has to do with people not understanding how scaling and map depth have to be for them. In Reach almost every 1v1 map other people made were 2v2 maps. This lead to a tournament having enhanced radar for 1v1 based off the reasoning you could hide on the maps. When in reality none of those maps were really 1v1 maps. I think that issue came from people emulating quake maps rather than emulating the quake gameplay.
My personal favorite thing about designing 1v1's is it is a great space for testing different geometry and gameplay ideas that are usually harder to squeeze into large player counts. Since you don't have to think about things involving teamwork.
Discombobulated work rant.