It's unhealthy and actively destroys the game by creating a situation in which the stat needs to do everything to appeal to all of the classes which utilize it differently.
AD scaling on all skills either needs to be removed or gutted.
If you want to have 4 AD scaling abilities, then any additional damage scaling should never be anything more than a secondary aspect of the kit and should never be allowed to be core to it.
If an AP champion gets behind, they basically lose all of their damage influence in the game, they are forced to resort to their mechanical influenced that are attach to their CDs, but this is gated heavily by substantially higher mana dependence on AP champion, which just gets worse if you're at 40% CDR. Where as if an AD champion gets behind in a game, their damage isn't made nearly irrelevant, just less influential, yet they retain just as much of their mechanical influence as an AP champion. But in the case that neither is behind, then there isn't any kind of advantage to being AP, they're on par at best. Making AD lower risk equal reward!
The normal argument for why this is normally has to do with AP champions tending to get more CC, but all of that is made irrelevant when AD champions keep getting designed with more CC and general mobility creep, since virtually all AP CC is still being attached to skillshots.
DIVIDE THE ROLES OF THE PRIMARY STATS RIOT. THIS ENCROACHMENT OF AD INTO THE REALM OF AP HAS GONE IGNORED FOR TOO LONG AND MIGHT HELP YOU LOWER GAME TIMES TO YOUR TARGET 18 MINUTES, BUT THAT'S KILLING YOUR GAME IN THE PROCESS. **