It seems that you are right that the Observation Post is missing its cover bonus. I will certainly add that. The Pantherturm is more durable though.

Bunker,
HP: 480
Armor: 35
Pantherturm,
HP: 750
Armor: 50

I believe the reason it's a lot more likely to go down is due to the fact it's a stationary emplacement with a large target size. Not any different from other emplacements from being a bombing magnet.

Without an MG to deal anti-infantry, it's not that hard for a Penal Battallion to drop satchel charges, a Ranger or Engineer squad to wire a Demo Charge, and practically has no stock method of dealing with an AT gun firing at it.

In my experience, yeah it goes down fast but that's primarily because of the resources you lost from it. Losing a regular bunker doesn't seem too bad of a loss but losing a big panther gun does.

What makes the Panthurm a lucrative option is that it gives the Axis an AT option that's invulnerable to regular infantry fire. It comes MUCH earlier than the Pak 43 but comes with a decent cannon to boot, that can also automatically rotate on it's own.

The Panthurm should never be placed in the front. It works best as a fallback AT screen that covers a wide area, since it can fire in a 360 degree radius, the Panthurm is a hard counter against mechanized tactics; something you won't see the AI do.

Against a normal AT crew, a truck full of Cavalry Riflemen could just circle around them and tear the crew apart, but the Panthurm stops that nonsense with its ability to constantly track and have the crew covered with concrete.

Against the AI, it's a great fallback investment when they inevitably start sending out light vehicles like the death truck, T-70 and Stuart. Just keep it farther back and it'll be difficult to dislodge the concrete AT guard.

You said it perfectly, thanks!