do they hit as hard?
does it have a negative effect on the sound quality?

they can hit just as hard.
as long as you have the right ratio for sub/box/car you'll be good and it'll bump

if you dont have space they are great
im just about to put 2 x cvt102 in my buddies silverado
memphis, jl, pioneer, kicker all have decent shallows

in a broad sense no they will not be as loud. regular subwoofer designs allow for higher excursion limits while shallow mounts generally have little to average excursion limits.

i'm not saying any regular sub is better than any shallow sub just to be clear.

as for sound quality i can't think of any reason why shallow mounts would have any different sound quality properties.

lol i put 1 shallow mount in a 48" long box with a missing sub in a shared baffle and it sounded pretty effing good to me, i Put the second in and it sounded clean but not as deep. I have no complaints about them,
the ctv's are each pushing 100rms above recommended and doing just fine.
they are specialist subs and when needed are awesome. Be interesting to get bunch and make a minimalist clam shell with cargo space still intact.