Pfusies don't really have one issue that absolutely makes them broken, but they combine a ton of features that basically no other squad has, especially not in combination. They are overall very easy to use and countering them takes way more effort than using them. Other squads with mixed weapons such as the Assault Guards that you mentioned have e.g. a clear weakness to vehicles due to the lack of snares. I can even drive an LV into them and expect it to win. If I drive an LV into Pfusies to keep them from closing in, I'll get snared. Same with mediums with either double snare or the omnipresent threat of ATGs
They should lose the AT Grenade and keep it as part of the Panzershrek upgrade.
Other squads with mixed weapons such as the Assault Guards that you mentioned have e.g. a clear weakness to vehicles due to the lack of snares.
This is something of a design failure for COH 2 that I would have liked to have been changed. Remove all vehicle snares from all mainline infantry and instead add them to dedicated AT Squads.
For example
Whermacht
Panzerfaust removed from Grenadiers
Grenadiers now have PZB 39 upgrade 40 Ammo, can fire a panzerfaust to snare vehicles
OKW
Panzerfaust removed from Volksgrenadiers when given any upgrade.
Raketenwerfer manpower cost to 210. Puts it on par with the M-42 45mm Anti-Tank Gun.
New Upgrade for Raketenwerfer
Rocket Propulsion Upgrade - Advances in Rocket Technology allow the Raketenwerfer to shoot further with more explosive power (Makes Raketenwerfer 60 range, puts it on Par with the Pak 40)
Sturmpioneers gain anti-tank snare grenade when upgraded with Panzershreck (no longer has stun grenade)
USF
Riflemen - Anti-Tank Grenade now requires Squad to have Bazookas equipped
LT - now has AT Grenade
UKF
Special Weapons Regiment (Commander)
Tank Hunter Infantry Sections removed. Infantry Sections now have Boys AT Rifle upgrade. Equips Infantry sections with Boys AT rifles and allows them to throw AT snare grenades
New Commander Ability added to Special Weapons Regiment replacing Tank Hunter Sections
Infantry Sections upgraded with Boys AT Rifles can now throw AT Satchel Charges like Penals
Soviet
Conscript AT grenade removed. Conscripts now have a PTRS upgrade package which allows them to throw AT Snare Grenade
Changes like this would need some tweaking but would generally improve the game. Units should have clear weaknesses, infantry included.