Purely for the first engagements I agree with you.
The last point though is quite a hyperbole. Vanilla pioneers can contribute until enemy mainlines get either upgrades or vet2, which makes them useful in combat for at least the first 5-10 minutes depending on the matchup. Especially in smaller modes where they are not focused out as much but can go more often into 1v1 situations.
Pioneer are SMG troops that can not attack past initial fighting and their best combat use is defending HMG from flanks, while having to fulfill a number of other roles including building base building.
From my experience it is rather rare to see a vet 3 pioneer.