I don't think anything is wrong with him in all honesty. Just people underestimate Y-wings in general as sluggish direct fire platforms and underestimate what a small turreted ship can do. The turrets in general all generally have a downside that may also affect the mindset: Autocannons have low attack and has range 1, blaster uses your entire turn, Ion is expensive and fairly weak in terms of sheer damage. Just Kavil makes them all better.
It also suffers a simliar pitfall to B-wings. They are not necessarily durable, just reliable in that they generally have to take a fixed amount of damage before they die that doesn't improve with points invested, especially if focus fired but on the flipside it is simply reliable and is unlikely to simply crumble to oppertunistic shooting. thus often why the 18 point Y-wing is preferred because it's the cheapest thing you can get with that robust frame.
That being said I just simply think that Kavil is over shadowed but not necessarily out preformed by Xizor and the Bounty Hunters. I just feel people simply overlook it because it's just a Y-wing that isn't exceptionally durable nor list dominating. Thus one has to be careful with how they use him more so then many other ships since he is very senstive to being blocked (if dps) or lacking power.
That being said, I consider him a fantastic ship and pilot, and by far one of the most impressive of the "starship series". Only Xizor and Guri is more noteable, though they lack the 360 view that he has or the sheer firepower he packs in a 2 range bracket.