I've seen basically 3 gripes.
1. The pickups are weak. Not in my experience
2. The preamp is weak- again, not in my experience, both of mine sound great.
3. They have a higher than average rate of trussrod failure. I know there are a couple guys on here who have had trussrod problems with carvins, but I'm not at all convinced that the rate is any higher than in any other brand.
My only gripe is that I don't like the wilkinson bridges they used to come with (both of my early/mid 90s ones have them). I get a little bit of saddle rattle on one of mine with super light strings. Unfortunately, the plate is so big that you can't easily upgrade them.
I usually don't commment on my experience with Carvin basses because there's alot of Carvin fans here. But I'll be as constructive as I can be.
I owned a LB 76 and a LB75 adn a 4 stringer for a short time. I sent the 4 string back after 1 day because it soudned totally dead. I was shocked.
The LB76 was a walnut body with 5 piece mahogony and maple neck. It needed about 1 full turn on the truss rod to acheive normal relief. And basically the neck kept bowing to the point where I ran out of truss rod room.I properly humidify and dehumidify my studio and own about 12 different guitars and basses.Not one of my other instruments has ever bowed like my Carvins.
My LB75 was a maple body with maple neck and birdseye finger board. This bass also had a neck that kept on bowing and bowing til the truss rod finally ran out of room. With both basses, it only took about 3 months before I ran out of truss rod adjustment. My feeling is that the lumber was green and not properly racked/ seasoned.
In both cases, the basses played well with the exception of the necks going boomerang on me. I also have had 2 carvin guitars.1 of them (7 string)was quite stable and the other one (6 string) has a neck that likes to wander.You'd think the 7 string would be less stable.
As for the sound, I think the Carvin pickups were only average. Not horrible but not on par with Bartolini, Lane P's and such. I did however prefer the tone of both Carvin basses passive instead of active. I felt that the weakest link on both of my Carvin basses was the on board preamp. Once activated, there was an instant mid scoop which muddied out the low end. The bass had more clarity and punch in passive mode provided it was plugged into a good amp and cabinet. So I never used the Carvins in active mode. I think a good preamp will enhance a bass's sound and the Carvin preamp seems to be voiced to sound very bassy.
Having owned 2 Carvin guitars as well and trying their various pickups, I can say that their guitar pickups are way below average. Many of them have a piercing high end and below average midrange definition.
Lastly, I live right near two veyr cool bass retailers and have ben able to try dozens of high end basses as well as mid and low priced basses. And I have run across 3 or 4 Carvins in these stores and 2 of them had neck issues. So a that point, I gave up on Carvin basses adn bought a Spector usa bolt on. Moved onto a Roscoe SKB 6 and then onto a MTD USA 6 and also own 2 Zon Sonus 5 strings. Never had any issues with any of those basses atleast as far as neck stability goes.