Zero
Master Black Belt
Its amazing how often I hear this, yet I still find plenty of karateka who exhibit pretty sub-par techniques compared to boxers and other "sport" fighters. Heck, I posted a few videos earlier in this thread.
Now keep in mind, in a vacuum, Karate hand techniques are just fine. However, if someone was asking me where to go to best develop hand techniques, I wouldn't suggest Karate to them. If they needed a good punching and kicking game, I would suggest Muay Thai kickboxing. Again, I simply couldn't recommend Karate in good faith.
Look, I have fought plenty of muay thai fighters and kick boxers and have found their hand skills no better than mine. If anything, I would say I am not overly impressed with the punching of a lot of muay thai fighters and personally have issues with their upper body/gloves fight and guard stance. When competing I am generally in a boxer's position, albeit with perhaps a slightly wider and more side on stance/positioning of the legs which enables more effective and quicker throwing of kicks but my hands are pretty much as per a boxer's, this is different to muay thai guys I have fought and watch.
I would also say that your comment that karateka have sub-par hand skills to other "sport" fighters is not correct, or at least very hard to justify.
In my experience, and having done TKD all through high school myself, karateka generally have better punches and a better defensive guard to punches than TKD guys - that's clearly a generalisation. Perhaps kick boxers are on the whole "better" than karateka but not sure if that is the case really (same goes for muay thai, keeping the elbows to the side of the equation). And aside from TKD, MT and kickboxing what else do you have sport wise other than boxing and karate (...maybe savate). Are you saying bjj and judo have better hand striking skills than karate in general??
There is a lot of suck-bad karate out there for sure but I honestly think you got this suck-bad training yourself in your years of Shotokan, either that or you did not put the time in or somehow didn't "gell" with the training and/or style (which happens), which you seem to have done when you took up boxing.