People like
Tanka mentioning how much "twinkiness" it causes, or that it should be disallowed out of "sheer principle" for the same reason. Ergo, they must think for some strange reason that it's unbalanced and worthy of banishment from the rules.
"Oh drats, I just paid 324,000 nuyen and 1.2 Essence for my Skillwires 6/108 and 10,800 nuyen for a Assault Rifles 6 skill... I'm totally going to cheat and pick up this here SMG and shoot you with it, albeit it with a +2 TN penalty, but whatever! I'm now the undisputed king of munchkins! Yay me!"
Where exactly does it become twinky? With ranged weapons? That +2 TN is pretty hefty and on par with shooting two weapons simultaneously, and then only with weapons at least somewhat similar to the one you're already using. Melee weapons?
Any penalty in melee combat is a very bad thing regardless of how many dice you're throwing, whether it's with a katana or an extendable baton. Vehicles? That +2 penalty is on par with having your Handling increase by 2, which is quite significant. Social Skills? The penalty is the same as if you had just increased all your relations from Neutral to Suspicious, or the request is Annoying. Technical Skills? Only relevant with Electronics and Computers, and then it's still a +2 penalty just like every other time. Magical Skills can't be defaulted to let alone chipped. And regardless of everything else, you had better hope the final TN isn't above an 8, else you can't even do it.
Do Chipjack Expert Drivers somehow make it unbalanced? At best, you get a single bonus die when doing it (max rating of 3, cut in half and rounded down). Whooptido. Adepts suffer the same problem with Improved Ability. And either way, you're still limited to defaulting to skills closely related to the one you're focused on anyway; it's not like you can use Pistols 6(12) to shoot a rocket launcher or drive a nuclear submarine.
I'm just not seeing it.