The weakest link of J2nt is the 1S 2N 4S sequence. A simple cure is just to use 3NT to show balanced minimum and 3S to show balanced, extra. Therefore, you still have a whole 4 level to cuebid. Those who claim it is a simple pass over 4M certainly never encounter a hand like KQxxx Kxx Kxx Kx, where 6NT is cold. Or QTxxx xx Kx AKxx, where 6S is very decent. It is indeed a systemic issue, which can often be solved. Of course, if you want to improve more, you can design some better structures. For example, 3C to show weakness, 3D to show shortness somewhere and extra, balabla. Or 3C to show shortness somewhere. 3D to show balanced, extra. 3H to show balanced minimum. Of course, later relays are needed for such structures. Still, Jacoby 2NT is a very useful convention if it is revised a little bit. It's just like Stayman. When 2C was first used as stayman, it was certainly quite different from what experts are using nowadays.
