Let's discuss the role-confusion that divers are facing

Preface A diver is someone who is somewhere between an assassin and a juggernaut. They are meant to be able to dive onto the target and pressure them in teamfights, either forcing the target out or forcing retaliation. They are meant to be balanced with damage and tankiness, and generally require highly reliable kits in order to perform their function correctly.

Core aspects of a diver include:

1. Reliable gap closer
2. Innate tankiness or defensive steriod
3. CC
4. Reasonable up-front and sustained damage
5. Relatively few reliable escape mechanisms (in most cases)

Some classic examples of champions who could be played as or considered divers include:

Many of these champs, depending on how they're built, could also be lumped into other categories. I'm not here to have an in-depth debate over who is and isn't a diver, I merely put this list here to serve as a basis for people who are unfamiliar with the role.

Current Meta

Many of these champions have had their struggles in viability from time to time. In the recent meta, a few (Wukong, Pantheon, Xin Zhao, etc) have found viability by completely ignoring tanky builds, and instead pumping as much damage into their kits as possible. They instead become semi-squishy assassins who frontload damage onto the target. When divers are instead played as assassins, this creates major problems for the health of the game.

Problems

1. Feast of Famine play patterns. Because a Diver generally lacks a strong escape mechanism, they are unable to flit in and out of fights like assassins and pick up kills. They also lack strong utility, and generally require close range to accomplish their goal. This means that unless the diver gets fed early, they will likely get blown up in fights. If they get fed, however, they have extreme high points, often feeling tanky because of their innate defensive steroids, and blasting through targets quickly. This is exacerbated in lower elos and with less team cooridnation, making them extremely effective pubstompers, but inneffective elsewhere.

2. Lack of meaningful counterplay Unlike assassins, who are (theoretically) built on having required skill shots and kits which you are able to play around (I do not want to discuss assassins in this post, but that is the intended play pattern for them), Divers are instead built around having extremely reliable damage. A classic example is Wukong. Nothing in his kit is a skill shot, or has any reasonable chance of being avoided. E + Q + R is all point blank lock on damage. The same is true of Nocturne, he will unload his damage and there is little you can do to prevent it.

The lever which is supposed to keep this in check is that Divers are not meant to be able to 100/0 targets quickly. They are supposed to build tankier and go for more sustained damage patterns, with initial burst which can make them a damage threat. However, because so much of the damage is disconnected from the kits (Stormrazor/Duskblade/Electrocute), their reliability is unchecked, and playing against an assassin-build diver is extremely frustrating, particularly if they get fed in a different lane.

3. Suicide-bomb syndrome This is perhaps the greatest issue. Divers are not meant to flit in and out of the fight. They are meant to dive in and be a continual damage threat. However, because they are building glass cannon builds, they more frequently end up diving in and either dying, or blowing up any number of targets and then dying.

Assassins are balanced around being able to go in and out of fights (riot has clearly stated as much). This is actually healthy; it means they are able to unload damage and then leave, and do it again. It creates a more consistent and balancable playstyle. Divers, on the other hand (unless grossly fed and with a lot of lifesteal) will dive in and almost always end up dead, but if they're fed, they'll take a few people with them. Being a walking suicide bomb is extremely unfun for the person doing it, and for the other team; you know you'll lose people to the diver even if you blow them up.

Case examples A few people demonstrate this problem painfully well:

1. Rengar Though he's almost always been played as an assassin, Rengar's kit was built to be a diver. He's listed as a Diver riot's page, and he has all the classic examples of it in his kit. He perfectly exacerbates all of these issues. From a skill standpoint, none of rengar's burst is avoidable. He ults, then jumps on you and kills you in under a second. The attack speed from his Q is entirely superfluous. He also has no reliable way out of the fight. Once he jumps in, he either kills everyone or dies very quickly. He's complained about constantly even if he's weak, because his kit has all of the above problems.
2. Nocturne Again, same issues. Nocturne has got a lot of attention recently because his ult + Q + AA combo kills people almost instantly and cannot be avoided. All of the above issues are in place: when he's fed, he's a nightmare, when he's behind, he's non-existant.
3. Diana She's a more interesting one, because she (theoretically) requires you to land a Q, but she's gotten some attention recently for her burst. Diana has the kit of a diver, but has been played as an assassin for a long while now (ap bruisers suffer in being able to itemize effectively). Her kit suffers immensiely from these issues, with frontloaded burst and inability to escape, a fed diana is a nightmare of resetting Rs that doesn't even need to land the initial Q to kill, and an underfed one is borderline useless. These issues crop up in almost every Diver on that list, because it is unhealthy to have reliable frontloaded damage mingled with innert tankiness and an inability to escape.

Possible solutions: (Tank itemization/masteries is a seperate issue. Though it would help immensely if tank options were more viable, I won't go into that in this thread)

1. Transition dedicated Divers to gradual damage rather than frontloaded burst. This would solve a lot of issues currently plaguing the diver class. Imagine, for a moment, if it took Rengar more than .7 seconds to kill? Or if Diana actually had to make use of her passive in order to burst someone down? Fighters would need strength elsewhere in their kit (perhaps more tankiness and greater ability to stick to their targets) but in order for them to fulfill their role in a meaningful and healthy way, they can't be able to burst targets down quickly.
2. Rework certain divers into assassins. While some champions are built around the idea that they can be divers, many have been played exlusively as assassins since their conception. With Rengar, for example, there was extreme backlash whenever riot has tried to transition him into more damage over time with tanky builds, even though objectively, that is a healthier approach. If this is the case for a particular diver, the better solution might be to embrace the assassin aspect and give the diver the tools to be a successful assassin.
3. Provide better itemization for AP bruisers While relatively few divers are AP, this is essential for them to be able to operate. Diana, for instance, just can't get to the level of tankiness of most bruisers because ap itemization doesn't allow her to. The same applies to Elise, whose base damages are harsh enough that she has to build damage to stay meaningful, but doesn't have the tools to both get in and get out of a fight.

Conclusion I believe a some of the major game-health issues in champions hang on this issue. Divers have struggled to find a space, so they are now built as assassins, which creates toxic and unfun play patterns. If Divers were balanced around consistent damage and threat with survivability, it would not only give them a more clear direction, it would make them more valuable in higher levels of the game and more enjoyable to play as and against.