I approached one of these companies, back in 2019, when I was let down at the final stage of selling my house. I had agreed a reduced price from original for this sale, and on the phone the company offered me something like 80% of that.
I did not go ahead, but understand that even with that low price, if you go ahead, it gets reduced further when they actually send someone round, who looks for every possible defect they can.
Suppose okay if you are selling a property which is virtually incidental to you - say a relatives house a long way away, but for most of us who are selling with intention of moving, so usually need every penny we can get - it is not a good idea.
Suppose a better option could be going to auction - at least there you can signify the lowest price you will accept.