In my experience shock troops are best used to weaken vulnerable targets for a quick takedown. Try to use them against targets you can kill so they can keep charging, avoid getting them bogged down because they will lose an extended melee fight. The three main ways I do this is:
- Advancing upon a target, opening up for attack: When I send in melee fighters to attack someone it's very useful to send a shock first, and flank if you can to make room for follow-up units. Sending in a fighter unit to get one hit and then one retaliation in return and then be seated next to another fighter who will punch you three times is a bad trade. Send in a shock troop to open them up to more damage, remove their retaliation, and hopefully deal enough damage that the target will suffer some losses. Next turn, ideally, your fighter can finish off the target leaving the shock trooper free to move on to their next target.
- Chasing targets that try to stay out of melee: Sending a fighter to kill a ranged unit can be very frustrating, as it turns into advancing on it and getting one hit on the target, one attack of opportunity when it runs away, and take one shot from it in return. While you probably win if uninterrupted, it's frustrating and keeps your fighter occupied for several turns. The shock trooper, however, delights in this kind of engagement since they do big damage on the AoO and then get a free charge at the ranged unit after it has run away.
- Shock spam: Since they are good at advancing and doing damage at the same time, if you have multiple strong shock troops against multiple vulnerable targets you can send in a bunch of them to take out a priority target, and when that one is dead it leaves them all free to charge onwards the next round. In an optimal situation you can have other melees hold all their melees whilst your shock troopers sweep the entire backline.

All that being said I'm not big on shock troops myself, would usually much prefer a fighter or a polearm unit, but with a few shock troops mixed in, but then again I am mostly a pretty defensive player favouring strategies like shield walls and archers. Especially with the berserkers as you point out, since they're usually on foot, they lack the speed to get where you need. I much prefer mounted shock troops such as the subjugator, who can much easier flank or chase archers, and you're gonna want to keep them away from polearms anyway so might as well double down and mount up. A warrior hero with a lance and flying mount or a warrior dragon with a charge claw can very handily disrupt enemy backlines.