Meh, using them on mid-long range engagements makes them pretty bad compared to Volks as their DPS is not that much higher on those ranges and they lose 25% of it per dropped model as opposed to Volks who retain their highest DPS weapon untill the last two models. If you want mid range engagements you're better off with STG Volks, and for long range there's LMG Obers.
FSJ have nearly 300% the dps of volks k98s at all ranges including long range, and 40-50% more dps than volks stg44 at short and mid range(then 3x the dps at long range).
In addition, FSJ have 0.83 model size out the gate compared to volks with 1.0. This gives the FSJ squad an effective small arms durability of 385.5hp per squad vs volks with 400 (or grens with 351.6hp). point being they arent really all that squishy, only issue being the small squad size vs aoe.
They are excellent units to use in conjunction with Valiant assault as that 25% dps boost on top of their already impressive damage output means they will mow down an allied squad in seconds if the enemy isnt paying attention.
Edit: max range squad dps vet 0
FSJ-20
LMG42 Gren- 15.7
LMG34 Ober- 25.5
Stg44 Volks- 8.3
G43 Pfusilier- 11.6
Non ptrs Penal- 10.8
Double BAR rifle- 13
Note- rounded to nearest 10th