It may be, but difference here is there was max two Water Bubbles and one good Water Absorb could check/counter both. So sure, it wasn't pulling its full weight outside of that, but you only needed one. A player can, however, have up to 18 instances of trapping attacks on a team. Now... that's ridiculously overkill, but 4-6 instances of Anchor Shot on a team seems quite plausible. And in order to nullify it, each member on your team needs a pivot, shuffle, or Ghost-typing. Otherwise those lacking it risk getting trapped against something they can't handle. Offensive or defensive Pokemon, they're at risk if they can only manually switch. And while U-Turn is ubiquitous, I don't find the idea of being forced to run it or other trap-escape options on 4-6 Pokemon to be healthy. When people say they always make sure every wall they have carries U-Turn, it raises a red flag for me.
...also, a lot of what I said there can be applied to Shadow Tag Gengar too for that discussion.
Anyway, basically, its 1 ability slot that can be dead weight outside its check vs multiple move slots or running specific Pokemon across your entire team. I'd say 3-6 move slots is a way bigger cost than 1 ability slot.
Also, as said, if you want some proof, well, here's some stupidity Anchor Shot can cause from the BH Open:
http://replay.pokemonshowdown.com/gen7balancedhackmons-557006042 Turn 57-135 becomes a pointless slap fight because both Pokemon Anchor Shot each other. Ironically, Shadow Tag doesn't actually have this problem. (Arena Trap still does AFAIK, however.)
If you guys really want, I can look into building a team whose sole-purpose is to abuse the living hell out of Anchor Shot. You'll make me hate myself doing it though, I'm sure.
As for more on Core Enforcer, well, not a lot for me to say beyond what I already said. It's insanely splashable with no drawbacks and very low counterplay. Same for Spectral Thief, but I care least about that one. Most about trapping.
I shoulda included this in my first post, but this is why these caught my attention.
-Anchor Shot and etc. hits the second bullet point and the third in spades. It's "countered" by U-Turn and such, but stuff like fast Encores or Normalize Entrainment Gengar to counter the counter, kind of the same issues STag Gengar has, except on potentially anything. The counter isn't niche, but I'd consider it somewhat unreliable since it can be shut down quite easily. Also Ghost, but as common as Giratina, Gengar, and Shedinja are, that's really niche.
-Core Enforcer hits the first one, since its counters are Fairies and going second, and third points quite well. Second one is less so, mostly on removing emphasis on planning/preparation beyond Fairy spam or running junk like level 95 Pokemon.
-Spectral Thief hits basically the same points as Core Enforcer for similar reasons. Counter is Normal-types, which are pretty limited to Gigas, Arceus, Slaking, and MAudino for the most part. You can't really prepare for it short of only running Normal-type set-up.
I simply see some major red flags, especially with trapping moves, hence, again, why I'm bringing it up.