Good list, I think you've rated them fairly well.
I think the issue with having a "very niche" weapon is that it's too niche. Flexibility is a stat people rarely take into account when talking about weapon balance. It's quite simple, why should I spend 300$ more on a Krieg if I can get 90% of the same kills with an AK? Same goes for P90, why spend 2350 on a rush weapon when I can either spend 1050 on a slightly worse rush weapon (MAC-10), or spend 1800 on a weapon better for holding angles (Galil)?
Niche weapons do have a place in the game, of course, much like the SSG 08. Unless you're hitting some insane headshots you're not gonna be using it the same way as an AWP, and that's why it's so much cheaper. It is, however, incredibly useful for tagging down opponents quickly. On a half buy or force, getting one or two players down to 30 HP can be crucial for getting SMG kills. It's a great support weapon, even if it is very niche.
Weapons like the Nova may be niche in the way of "only useful for close range", but there are other weapons that do that and more for a similar price tag. Example: You're playing the apartments balcony on Inferno. Your responsibility is to hold the aps-push, and maybe short. The gun is great for that. But say they take B? Whether you're engaging from Banana or CT Spawn, you're at a disadvantage against SMGs and rifles. If you instead bought an MP9, you might not be able to dish out the same one hit headshots in aps, but you can still get the frags. And on the retake you can take the gun on the offensive and it's still a viable gun.
What I'm saying is that some guns need to have their own "gimmick" if you will, to actually make them useful. I don't think this was kept in mind when classic CS was designed 20 years ago, and they just threw ideas at the wall and saw what stuck. Since then it's been roughly like this, and it hasn't changed.