Iirc Shreks are more accurate than zooks. Also when's the last time you saw a Shrek bounce?
To avoid being overpowered against light vehicles in the early game, the sturm schrek was given poor far accuracy, as I recall. Unfortunately, light vehicles are the only reason I would ever build one.
Every time you tell them to shoot they do some sort of dance (a squad AI bug maybe?). By the time they're ready, the vehicle is out of range or it's running into the squad so they can't do anything. And at one shreck per 300MP 4-man squad, they're useless against real armour since schreks do far less damage than AT guns. Who is ever going to build more than two of these squads? Anti-inf loss aside, if you do engage, the time it takes to actually threaten a tank means you can easily lose a large amount manpower. There's a reason Pgrens get two of them.
By comparison, bazookas shoot almost as soon as you tell them to and can be equipped twice on every unit in your army. That means that even with bounces, they do real and relatively consistent damage. At the very least,a few zooks will make a medium tank back up.
Double schreks or just some new tank hunter squad would go a long way in my opinion.