ugly spot :/

Given there are no flushdraws on the flop and almost all straight draws get there it indeed is hard to find a bluff he can do this with on the river. If he's decent then he might turn 88 or 99 into a bluff on the river because he shouldn't really ever win against your flop check-raising range but probably he's squeezing those with at least some frequency. He could also decide to turn two pair he called the flop with into a bluff.

If you think he could ever valuebet a set here then this should become a pretty straight forward call.

If not it really comes down to whether you think he will turn some two pair into a bluff. If he finds the perfect frequencies I agree that PIO should spit out a call here given your exact set combo also blocks his 76 combos.

Interesting hand. Preflop all seems very standard.

On the flop this is a board where UTG should be almost range checking three way vs BB and Bu on this board texture. When UTG does decide to bet then I would imagine raising is the highest frequency play, but certainly flatting would be mixed. Now when BB cold calls your raise I would imagine them to have the occasional overpair that didn't decide to squeeze (but these would most likely fold flop without the backdoor straight draw), also sets/2pairs and straights. Depending on the player/player pool you may seem them call hands as weak as a7s (with the BDFD).

Now on the turn you are most certainly not capped in your range in terms of having straight (you will occasionally raise flop with 67s combos/77 and I imagine the occasional a7s with the BDFD. But in saying this most of your straights on the turn will check back to protect your weaker value and bluffs.

So when we get to the river and villian bets about 80% pot, I would be inclined to think that most villian's will not be betting a set for this sizing on the river given you can still have a strong set/straight. The same goes for 2 pair combos and majority of villains are most certainly not turning these into a bluff.

IMO PIO would say 66 is a pure call, but also population isn't going to find the necessary bluffs in this situation and because of this I would imagine the calling frequencies of 66 to go way way down. I fold this spot due to population tendencies and not worry too much about it as we have a bunch of straight we can happily continue with.