Both of these vehicles were never designed to have the m40 recoilless rifle mounted, and as such when it was done, the m40 was simply mounted on the roof, without any internal controls arranged.
Additionally, you can clearly see in the in-game model that both vehicles have both the horizontal and the vertical controls positioned outside the vehicle’s hull, so that the gunner would have to expose their upper body to control it. Using the sights on the m40 recoilless rifle would also be impossible to do without partially exposing yourself on both vehicles, as they clearly have the gunner’s sights positioned on the barrel.
The SPz 12-3 LGS also has an adjustable mount for the m40 assembly, where someone could slide it forwards or backwards. This would also further complicate operating the m40 rifle controls from the inside, as then there would clearly have to be a slit in the roof of the hull for the mechanical links, or any potential electrical equipment to move with the m40 recoilless rifle assembly.
Now would this break the tanks?
My opinion is that it would not. Both of these vehicles are meant to be played as lightly armoured manoeuvrable vehicles, meant to flank the enemy, or use hit-and-run tactics. But what actually does happen in war thunder due to this inaccurate and unrealistic gunner placement, is that players use them as hull-down beasts, as the hull cannot be directly hit if the player simply pokes the m40 recoilless rifle over a hilltop or cover, but not the hull. So obviously making the gunner partially exposed by putting the gunner’s upper body next to the m40 assembly-controls would mean that the gunner would be very exposed to enemy fire, which should not be a problem if the player uses the vehicle the way it was meant to be used, and it is only the gunner that is exposed, meaning that the player can still drive back into cover.
As always, thank you for reading!