NO SPOILERS. Unfortunately most of the negative reviews are from people who didn't pay attention, or wanted a hand holding jump scare. Some just didn't like the ending and that's OK as it is polarising.
I loved this film, the casting was superb as was the acting, script, and research. Every character felt human and made realistic choices true to their nature.
This was a slow burn which ended with a big bang. I would say if you like Midsommar then this may be your thing too. Although in fairness this does not come that close to Midsommars level. Things seen and heard is a fantastic psychological thriller with elements of horror and I give it a 9/10.