He's still more successful as a tanky bruiser in spite of the base damage nerfs, as opposed to a conventional AP assassin he was designed as. He's prone to getting counterjungled by other champions who can throw their weight around and pester junglers with relative safety, and with that chunk of HP and resistances he stands a much better chance of not being blown up instantly and having a lot more wiggle-room as far as self-peeling and slipping out of an unfavourable fight goes.
Most assassins are a niche jungle choice in general because they don't have the peeling/initiating power or the same ability to commit to a fight as most tanks/fighters do, and are tuned toward blowing people up in the earlier portion of the game and maintaining a snowball that jungle camps alone can't provide. AP Ekko is a shining example of this, and when a (sort of) off-meta build of his in an unconventional position starts to fall out of favor it is is unrealistic to expect him to maintain the degree of sh**roll-factor he previously had.