It almost feels like even the concept of a "diver" is an impossible class to balance.
If you think of say a classic mage, your goal is to cast your spells from far away and do big damage to typically a big aoe. (Just in general mages, not just LoL mages). So if you are failing as a mage it means you are doing less damage, and if you're succeeding you're doing lots of damage. This is easy(er) to balance because you just have to balance around how much damage they do, how fast they do it, and how easy it is to do it.
With a diver your success vs fail looks really difference. A successful diver can run past all their defenses and kill them despite trying to run away or hide under a turret, which feels awful as the player under the turret. "Even though I caitlyn trapped him, used both summoners, thresh flayed him away, and I cleared the whole wave so he had turret-aggro the whole time, the hecarim still ran through everything killed me and got away with 80% health. No fun for the target.
On the flip side a failing diver is essentially useless because you are going to use all of your cool mobility tricks to get to their carry, and then just get cc'd and die without doing anything.
The difference between our hypothetical mage and our hypothetical diver is that a semi-successful mage still has a purpose, whereas a semi-succesful divers purpose becomes "I am not actually doing at all what I'm supposed to so now I guess I build tank and just be a less good tank?".
The fact that there is no neutral outcome for a diver is the main issue here. You can't dive someone, and not necessarily succeed, but not fail either. One of you has to die, that's the characters identity.
I realize this is just analysis of the issue, without any solutions, but... Just my 2 cents :P