I honestly don't think they are overcosted. They're very powerful and can soak up mistakes pretty well, making them fairly easy to use.
I think the trap that people may fall into when trying to get value out of them is either picking too many upgrades, picking the 'wrong' upgrades or simply not using the add ons in the right way. The frame itself will do work, if you're adding another 10-15 ineffectual points to it, then it's doing major harm to the efficiency of the rest of your list and feels like poor value. 3 Delta's in a list would be incredibly strong.
Saying that, it might be that Vessery is slightly overcosted compared to the others. With the basic price, it's not terribly easy to make the most of him through squadmates. To me, they're best as independent ships.
I do kind of think that Rex is definitely not 2 points 'worse' than Ryad as well, their cost could possibly be reversed. The I5 makes a fair difference. Small margins though.
Just as an afterthought, having seen/heard it mentioned a few times, an I6 Defender would just kill everything. I don't think you'd even need a pilot ability. I'd be tempted to run it solo with all the toys