Yes the demo was frustating to play against. So is the teller. getting one shot without warning or counterplay is bad including when ost does it. So defending the teller while supporting the demo nerf is very hypocritical imo.
Demo charges were much more cancerous than Tellers. It was a good thing they got nerfed. As said previously, you can protect your vehicle with a sweeper nearby. You can't protect all your regular infantry with sweepers unless you want to go for engineer spam.
Teller mines are as strong as they are because Ostheer has no real LV and 4 man squads that are vulnerable to vehicles because of high reinforce cost and overall poor survivability.
IMO Teller mines are balanced. If OKW had them they would be OP I agree. But for Ost they are ok.