I've been staying at private rentals since long, long before Air B&B existed, and Air B&B has never been my favourite platform to use, although I do use it a little more lately.
I'm not influenced in my reviews by the host's ability to review me. (Other platforms, such as VRBO, have this now, too.) The hosts don't get to see my review until after they review me, and vice versa.
I HAVE, as others have said, messaged a host privately about an issue, if it is something easily resolved and not intrinsic to the place, and if everything was otherwise good.
In reading reviews, I find so often that negative comments are petty things that don't bother me at all. There are some deal-breakers, and I will search for those. There are also instances in which it is obvious the reviewer is carrying out a vendetta against the owner or hotel, often because they were asked to be quiet or something.
Petty- the king sized bed was really two twins pushed together. So???? That's common in Europe and not a problem. In fact, now that my husband and I have motion-beds, that's our setup at home, too. Who cares? Why is this a problem?
Petty-the toilet was in a separate room from the shower. So???? This is also somewhat common in Europe, and it means that one person has access to the toilet while the other is showering. I see this as a benefit. Why is this a problem?
Petty-the stove was just an induction hotplate. So? I'm not travelling so that I spend my days in the kitchen cooking big meals. It's a bonus to have a bit of a kitchen, for when we have an early start and want a quick breakfast, or we want to do takeout or leftovers or a quick meal at the end of the day. I love induction hotplates, and it was having one in an apartment in Budapest that led us to buy an induction stove.
Petty-we had to turn on the hot water before we could shower. So? Again, this is not uncommon outside of North America, and it is much more efficient and environmentally friendly. A tankless, on-demand water heater also means not running out of hot water mid-shower.
Deal Breaker - #1. Bedbugs. No way. No how.
Deal Breaker - if the owner has frequently cancelled reservations at the last minute.
Deal Breaker - really bad wifi. This depends on the situation. In Africa and in the Galapagos wifi wasn't always a given or was available in common areas only. These locations were quite isolated, so that's to be expected. In an otherwise wonderful B&B in Venice, the wifi was bad in the main room but good in the spare bedroom. The host warned us about this, so it isn't something we criticized.
If there's something I think could be problematic but wasn't for us, I'll mention it but not remove a star for it. For instance, if there are a lot of stairs and the place wouldn't work for someone with mobility issues, I'll say so, and I do appreciate knowing about such things in advance. That's not something to give a negative review about, though, in my opinion. If someone wants to stay in a heritage building in Europe, and not the Delta, then that's to be expected. I assume there's not elevator if an elevator isn't mentioned in the description.
When I read reviews, the ones in which I give the most credence to negative points are the 3 and 4 star reviews, where the traveller is clearly offering a balanced opinion and has reasonable expectations. Rarely is a place really as horrible as 1 star reviews paint them. That said, some places really do warrant 5-star reviews.