I don't own a Sport Boat, but I am constantly on the look for one. A car-toppable, seaworthy boat for protected saltwater or Maine's big lakes that can travel fast with a small outboard is a rare thing. And you can paddle or pole a Sport Boat through thoroughfares between lakes that are too shallow and/or narrow for a boat like my 14' Lowe--which needs a trailer, handles waves less well, and needs a bigger motor.
There is no perfect boat, or most of us would not be on this site, but for what I do with a boat, this would fill a lot of niches. I can't think of any other car-toppable boat that does the things a Sport Boat does. Some other square stern canoes are similarly sea-worthy, but most of those are too heavy for practical car topping. Light aluminum john boats are a lot less capable in waves, as are small flat-bottomed aluminum boats like my Lowe--which is also needs a trailer and a bigger motor to get on plane.
The comment about them being unstable is the first time I've heard that complaint, and I have a lot of friends who use Sport Boats to fish, hunt, and work on big lakes. I have not spent enough time in them to test that concern.