Except you won't use it all the time using this in a very good spot where you expect the enemy to try and just get in a shoot-out with Guards, there are counters to the ability. Being immobile is a major drawback, especially against grenades and if you break the ability, you're not going to be able to use it again in the fight.
It also does not protect them from suppression unlike Conscript Hit the Dirt, so you cannot use it to assault HMGs which are a strong counter to a unit that needs to be still to use all its weapons.
The lack of cooldown for exiting Hit the Dirt is so the squad can at least try to reposition and remain in a fight, rather than needing an instant retreat from the many grenades Axis squads carry.
The following is simply my personal opinion:
I do use the ability a soon as it becomes available.
Being immobile is a drawback on the other hand it remain far more powerful than medical kits, tripwire flares etch...
Trying to close in to "hit the dirt" Guards to throw a grenade is not a great idea since one will be fired upon from ranges up to 42.5 and they can throw grenade back on the attacking path.
The ability has not no reason to be toggle since guards can be left behind green cover on hit the dirt to defend with no macro and it does not really need the extended range since PTRS already have 40 range.
In addition if one wants to create such a powerful ability the bonuses should scale with veterancy and not create a power spike at vet one.
On the Guard squad themselves, I would not mine if they specialized into either AT or AI, me and Smith had that set in a mod prior to DBP, but that never came to pass. They are not the most broken unit, though, nor do I consider them one. The main thing which makes them good is versatility, allowing them to put pressure on LVs and infantry.
The following is simply my personal opinion:
They are not a broken unit but they are OP and that is why commanders with Guards are mostly chosen. When trying to increase diversity instead of nerfing the unit the solution chosen was to buff shock troops which can create another set of issues. The units should simply become cheaper and toned down, the original design of "jack of all trade master of none" was allot better.
They trade very good at long range, where axis infantry are designed to excel, while they can cut down most axis infantry if they try to close on or bleed them in approach and simply retreat.
With the recent nerfs to mortars they where indirectly buffed.