Most of the customed heroes are very useful, a few ‘worse’ than the original version.
However, even if Chef Boldtusk a downgrade most of the time, I would thankfully take the stat and mana speed boost for the original hero in case I should ever draw him.
I fully agree with the second part of your statement. There should be more ways to get costumes without spending gems. It’s one of the major updates of this year and the vast majority of the player base are F2P and C2P players who cannot just throw out several 10x summons.
As for the hypothetical costumes for Atlantis heroes or (old) HotM: Contrary to most here, I wouldn’t mind and even welcome it as long as all players have a reasonable chance to get the base heroes (either TC upgrade or via Hero Academy). Also, if in future there are, say, 30 different 4* and 40 or more 5* costumes, we would definitely need more ways to get them apart from spending gems. Costumes for heroes locked behind the paywall, however, would further widen the gap between big payers and the rest for obvious reasons.