they don't turn well, they don't reverse well, they use a lot of fuel, they are noisier than a prop drive, they require a little more maintenance on the foot, slow speed maneuvering is mostly useless, can't turn without using the throttle (or can't turn very well), slower top speed, yes if you need a 25hp you have to get a 40hp motor and adapt a jet to it, which reduces the overall power to somewhere around 30hp. I don't know how other motor builders are but if you order a yamaha jet-specific motor, it is rated for the actual jet foot power (a 40hp powerhead will have a 30hp cowling but the powerhead makes around 40hp). The ECU for a jet is different than for a prop. Idle fueling and air and spark is different, among other things.
I have seen maybe 4 actual 25hp jets. A 25hp powerhead with a jet foot is about 18hp. One of those was a yamaha 25 twin cyl/twin carb 2 stroke. Talk about noisy. It went by me on the river and I thought man is that guy ever gonna let off the throttle? Well...I think about 25 minutes later he finally got so far away that it was inaudible. He was going maybe 10 mph upriver in a 2.5mph current, full throttle the entire time. Another one I remember was a mercury 25 with a jet foot. Still really noisy. The 4 stroke stuff is much quieter but still a lot louder than a propeller driven outboard.
but if you're running a shallow gravel stream/lake/river, with the right hull they are awesome
Everything else, a propeller is usually a better setup.
Everyone I know who has one loves it about 20% of the time. 80% of the time they hate it. Most of them are professional guides and they're mostly required to get their clients where the fish are at
there's been about 10 times in 10 years I wish I had one. Of those maybe 10 times, about 8 were to get over really shallow areas (less than a foot deep). The other two times? Other boaters who tend to fish too closely....with a jet, tilt it up a little, hammer down. Oops, sorry did you get a little wet? Power tilt is best for that....