The gamemode it's to be played like an aram. Champions that are good in aram are good in this mode, so "normal" team comps are moslty bad since, because of it's design, the howling abyss heavily favours ranged poke and waveclear over actual fights (unless both team go full melee), also the team with an advantage will snowball it easily thanks to the fact that, even if you die, you will get a lot more out of just one of your deaths than the enemy team out of the 3/4 deaths it takes them to kill you (since you're ahead and can buy one whole item, while they are stuck building components and sometimes can't even build those if they are waiting to just finish the item).
Even with normal team comps, there are champion that far outscale others in that map and some playstile that can be made possible mostly because of the peculiarities of the map, so maybe your team has sona while theyr team has nami (which I find a fair match-up on sr): sona can go full ap and not only do shittons of damage, but also can provide her ult's cc while nami is simply not made to be played full ap, you can try but it's not that good even in aram, so an even match-up on SR, becomes biased towards sona on HA. The map has just a different meta that doesn't really follow SR because of the peculiarities of the map itself, so it's not hard to believe that two fairly mathced comps on SR, are heavily unbalanced on HA.
On a final note: the mode is not to be take too seriously though, it's a silly mode so just do silly things that seem fun and actually have fun, don't think too much into it or it seriously becomes boring. Obviously that's just my opinion on it.
