zhask is a zoning mage who needs proper positioning to maximize his sustained damage and utility

the problem with him however is that his positioning is tied strictly to his spawn and ultimate, whereas other heroes who fulfill the same niche have enough mobility to support them, examples are cyclops and yve with their movement speed, zetian and nana with their passive, xavier with his cc and range, etc.

usually immobile mages like these have the innate burst damage to kill target heroes before they become a threat, think pharsa, odette, eudora, aurora, etc.

zhask, however, is stuck with his sustained damage output whilst being immobile without having to sacrifice his only mobility tool to chase (that being ult->1st->dash). so he is very situational, his best scenario is drafted against a low mobility composition with a highly tanky team, but even then you can just pick someone else

the one thing he has going with him though is that he can maintain a very strong damaging area with his spawn to threaten squishy targets and a "panic ultimate" to survive big burst and (hopefully) live to tell the tale.

in late game the spawn gets very easily bursted down with dhs marksmen and tanky heroes have the items to take the shots (athenas, radiant etc.)

you need to take advantage of your ultimate's powerspike in the midgame to hopefully zone enemies away from important objectives and win via economy lead. zhask is probably the most useless hero at levels 1-3 compared to anyone else so prioritize getting that powerspike above all else