A GTX 1080 ti is VERY cost effective on merged folding - probably the best card to use for that - *IF* you have a folding-optimized rig.
On ZEC, they're very close on hash/watt vs ANY member of the GTX 10xx family when all are operated at "most efficient" settings, and they've usually been very close on a "rig-level" basis on hash/$.
THE most cost effective, no - but not all that far off it and some folks are willing to pay a small premium for the higher rig density they offer (and lower time spent managing those rigs on LARGE farms).
The numbers probably vary if you're not in the US - some areas might charge more of a "premium" for the higher-end cards than we usually see here.
They are cool cards for sure but for mining purposes, the only thing that matters is cost efficiency and they fail pretty hard there.
No, they do NOT fail hard on cost efficiency - at the RIG level they are pretty much to a TOSSUP with any other GTX 1070/1070ti/1080 mode - unless you are defining "fail pretty hard" as "more than a percent or two difference".
Sounds like you got high rig cost problems. With the plethora of mining specific hardware available these days, anyone can get their slot cost down to at least 25, including power.
Its around 15% worse than a 1070 in terms of cost efficiency which in turn is worse than the 1060 and not to mention all the polaris cards. I'd call that failing pretty hard.
$25 WITH power is NOT a reasonable number, unless you are using total JUNK for power supplies or used parts - but even the BARE CARD figures on the 1080 ti keep it close to the other GTX high-end cards on cost efficiency unless YOU are getting charged insane amounts for the 1080 ti compared to the others.
SHOW ME a system that manages $25 per slot with NEW parts - I really would like to see that.
Then show me what kind of crazy rip-off pricing you're seeing on 1080 ti cards vs 1070/1070ti/1080.