tl;dr: he’s too good against certain heroes but too weak against other heroes. You either completely stomp Naz, or Naz completely stomps you.
What are the positives of Nazeebo? He deals the highest damage out of all mages. When going full Q talent, a single Q on an enemy will deal more damage than an entire KTZ combo, and same goes with all 5 toads landed on an enemy. He’s also tanky enough to be able to take a lot of damage, as well as having access to Ice Block.
Now what are the negatives of Nazeebo? No mobility, no hard CC, no burst, literally every ability is a skillshot. His Q will deal the highest damage in the game if the enemy is standing still or walking at normal speed; problem is most heroes can dash away or activate their MS. His W and E are very telegraphed; any hero that are fast can dodge his walls, and a lot of other heroes can free either themselves or their teammates from their walls.
See the theme? The more mobile you are, the better you are countering him. As if it’s not mentioned a million times that mobility is the most important thing in this game, here you have a hero that punishes anyone for being immobile. He’s effectively a hero that turns the enemy’s mobility into their defense.
One scenario is Nazeebo vs Sylvanas. You trap her into a wall and dump all your abilities on her. She haunting waves away and takes only 25% of her total health as damage, comes back and fights you while your abilities are on cooldown.
The other end of a scenario is Nazeebo vs Cassia. You trap her in a wall and poof she’s dead. She has no escapes, no spell damage mitigation, no ice blocks.
This is why Nazeebo is a godsend in lower leagues, but close to unplayable in higher levels of play. Most lower league players don’t dodge, don’t stutterstep, eats spiders and toads for breakfast, or waste their escape abilities willy nilly. Most lower league players also try to 1v1 Nazeebo (i.e. Artanis vs Naz) only to realize he’s too tanky and Naz deals way too much damage back to them. Players always underestimate his damage thinking “how can a cute little voodoo man hurt me” without realizing he has the damage to 1v1 even a Stitches.
Higher league players can dodge, can save their escapes, ice blocks, ETC knockbacks, or save their allies. Higher league players can take advantage of his slow damage, lack of hard CC, and overly telegraphed ability. Oh he wasted his wall? Time to jump on him. Oh I have spiders on me? I’ll just dash away. Oh he’s throwing out his toads? I’ll just move away or to the side.
He got a buff last patch but I personally think it did nothing to address his feast-or-famine playstyle. If you hit his abilities he’s better, but a smart opponent can juke you easily. In many ways Nazeebo is very similar to Kel Thuzad - high damage, but skillshot based, only difference is his damage is more DoT based.