The fact that there are a limited amount of things you can do to circumvent or skip a hero in the crawl, means that for the 2-ish months people actually grind the cavern, the match quality drops a pretty significant amount
I wish there was some stats on what the average game looks like before and during the release of the Cavern Crawl. I know that I have had quite a few games of people struggling to play heroes like Arc Warden or Wyvern. I genuinely feel the quality of my games dropping in this time-frame. I guess there isnt really a good way of measuring "game quality".
I understand it is a fun event to do, but I can't think of any other time a game company actively implements a feature that just directly drops the quality of the player experience for a limited duration. I'm not really complaining, I just wish there was a bit more you could do to either get rid of heroes, or at least complete difficult heroes vs bots.