Of course the UC can kill infantry. That's literally what it's for.
Then why it shouldn't be able to finish off/kill infantry units?
UC can't really wipe infantry unless you mess up and if you do it's on you not the UC.
Even if you do not completely wipe the enemy squad,you will bleed it and over the time you will cause more damage to him than your UC is worth.
You have to pick your fights, retreat if needed not to suffer any bleed.
And there is the problem. While Küble is easily damagable due rifle fire the UC can take quite a lot of damage and thus you won't be able to fight him. So you either need to fall back to your base and thus loosing map controll or you sit behind green/yellow cover waiting for your raketen to arrive while loosing models and, again, loosing map controll because a unit which is under attack and sits in green cover won't be able to cap points.
Just avoid it
How I am supposed to avoid it?
OKW player that doesn't use raketen vs UC or clown cars in 99% cases pays dearly for that. Don't be greedy.
The problem is not getting a raketen but how to deal with UC or Clown car during the time where your raketen is in building process and travels to the front.
And after you get fausts UC is not that big of a problem.
When you have fausts the UC will have Vickers or WASP upgrade. And even if not. You still need to get close enough.