As far as I am aware, all semi auto's have a ceratin amount of unsupported chamber.
The .40 is a hot load at around 30,000 psi which seems to be the cause of the few KB's, especially with reloads. The .357 is around 40,000 psi, but has much stronger shell casing, it is not just a necked down .40, but still has a little bit unsupported.
I've fired 5,000 rounds through a G23 and never had a single problem, I also have a .357 barrel for it and have had no problems with that either.
KB's happen with many types of pistol, it just seems that KB's in .40 Glocks have been documented heavily. Nearly every KB recorded in a Glock is the result of using reloads.
I wouldn't worry about it, the Glock in .40 is an excellent choice. The 23 is a good size pistol, big enough to be easy to handle, but small enough to carry easily. The later ones can be converted to .357 by dropping in a .357 barrel.
If you are concerned get a 23 or 32 and get both barrels. You'll enjoy the gun whatever you do.