In my opinion ALL mainline infantry should have the ability to build sandbags. The advantage of taking ground and digging in is negated when you cannot dig. Cover utilization and using infantry to take and hold ground is fundamental to the way CoH is played.
Not every infantry unit should have it though, just those that are considered “mainline” or “core” infantry.
In COH2 that means Riflemen, Conscripts, Grenadiers, Infantry Sections and Volksgrenadiers. All five of these squads should be able to build simple sandbag walls.
Additionally, Ostruppen should be able to build sandbags because they are essentially just crappier mainline infantry with no real advantages except cost, unlike Panzerfusiliers and Penal Troops who are an alternate to regular infantry, but do have specific strengths.
Lastly, all basic worker units should be able to build sandbags, barbed wire, some form of tank trap (Which shouldn’t provide cover, just halt movement) and some sort of mine. These are basic principles of modern warfare.
1) it mess with map design
2) it creates the need for rush the cutoff strategies
3) it creates snowball affects
4) it reduces diversity since there is less reason to built anything else than mainlines infatry.