Abathur has guaranteed and high value because his skills do a lot of damage, are high impact, global, and he can soak multiple lanes at once. This means that as long as he is alive, Abathur is providing a lot of value to his team and a good Abathur will die 0-1 times (more often than not 0). For all of the value Abathur provides he even ends up having the most damage in the game or close to it very, very often and if not him his primary hat target.
Abathur has insane synergy with too many heroes. He is a strong combo with characters who have some combination of damage, sustain, and mobility. That is so many heroes it’s ridiculous specially when you consider his clone ultimate which is even more guaranteed value. When Abathur clones the right hero (at appropriate times where they can’t just decide to walk away such as during an objective) he can just yolo dive the backline and either he gets a free kill or they waste a lot of valuable cooldowns. Either case Abathur wins because if you did kill his clone then his team is free to dive because his enemy team wasted too many abilities/hp for basically nothing. Abathur can just hat his primary hat target and you get nothing besides a crappy amount of XP, it doesn’t even trigger a reset for heroes for whom it matters.
Because of his insane synergy with other heroes, his balance is intrinsically tied to others’. This means Illidan can never be buffed even though he is trash because they know that Abathur + Illidan would be way too opressive. Which by the way, what is even the point of Tassadar when you can just pick Abathur and do his job better, safer, and with more damage. And just like he invalidates Tassadar, he invalidates TLV because unlike TLV who give up team fight capabilities to soak, Abathur is a monster at team fights and can soak almost as well, giving up nothing except being weaker pre-level 10. However, even that is not a real weakness because past updates have almost all but invalidated the early game. Late game comebacks are now ridiculously common, meaning it does not matter that he is weaker in the first few levels since you are all but guaranteed to make it to the late game and with his soaking capabilities it will probably also come with a level advantage.
Additionally, some of Abathur’s value is harder to see in the stat screen since the damage he gives to his allies through either sustain or hat buffs is shown under his teammate’s stats, meaning he has even more hidden value, similar to armor damage prevention. None of this is even mentioning his M.U.L.E. which can just passively counter entire objectives of some maps for free! Several minutes of fighting trying to win the objective completely undone without Abathur ever having to be in danger for even a second or even having to think beyond “oh that fort could use some healing”. And no, killing the fort is not counterplay. If a team had the resources to kill a fort they would do it regardless of M.U.L.E.
Want to try to kill Abathur? Well, you better be playing 1 of like 3 heroes who can even threaten him. And even then, most of the time it is not worth it to hunt him down. The time you spent going to his lane is time you aren’t getting XP or getting a camp and that’s if he didn’t just burrow away and make you waste all your time because his mines can cover every single angle and stop the gank. Now let’s say you somehow managed to kill him and his team wasn’t around to kill you while you dive behind the enemy fort. It still isn’t worth it because, unless it is late game, by the time you hearth and make it back to the lane Abathur already respawned and burrowed back to the lane, making it back even quicker than you depending on the time he died!
Abathur’s complete and utter safety takes away any kind of meaningful counterplay from the enemy team. It is time Abathur has some meaningful counterplay added to him. It is horrible game design that he is completely asymmetrical and you as his enemy can do nothing but hope to kill his primary hat target or hope the Abathur has absolutely no idea what he is doing. I hoped that his rework would do this, but nope. Then I hoped that the XP globes would do this, but nope they actually promote him being even safer than before thanks to his hat soaking taking over body soaking. And for god’s sake why do forts reveal stealth? This benefits no one except Abathur since unless you are blind you can see every single stealth hero while stealthed a mile away. It’s almost as if the developers don’t want you to be able to do anything to him, ever. He is safe space, the hero incarnate.
Just to make it clear, this post is not specifically about Abathur being OP (although he is). It is about the complete lack of counterplay available to his enemies. You as his enemy should be able to do something to Abathur, for example killing his clone should disable him from hatting allies for a few seconds. If Abathur was the worst hero in the entire game not much I said would change because you as his enemy do not ever interact with him, whereas due to the global nature of his moves he is always able to do something to your team. As it stands Abathur plays his own little game through a one way mirror.