I would say firstly, that the company selling them are always going to say they are great for obvious reasons.
I have a bumbo seat for my little one because he was taking a long time to sit up compared to others.
It didn't bother me because I knew he would do it when he was ready but I wanted him to sit in something upright to help strengthen his back.
It turns out the reason why he took longer to sit up was because he was born a big baby and its takes them longer to carry their weight and strengthen their body.
I would never of kept him in it for longer than 15 minutes at a time anyway, most babies get bored of being in one place and mainly because I know its good for him to just move around, explore and do what he feels is best.
I had my bumbo seat from when he was 6 months (now 9 months) but he used it about 5 times, again because he was born a big baby (and he has his daddy's legs lol) he couldn't get his legs in.
As I said I wouldn't leave a baby in them for more than 10-15 mins twice a day but they are good for using with the tray if they are on finger foods or Baby led weaning.