Thank you for actually attempting to understand the intent behind the question 'Why is their veterancy so bad?'
What it FEELS like to me is that they can compete with units in the early game, the second midgame rolls around, they struggle with everything except cons. Until cons reach vet 3. I guess its the weapon upgrades that make it feel like this.
By comparing them to other factions engineer units, is not a fair comparison, because sturms cost as much as penals, and are expected to be a front line combat unit. Why would you ever relegate a unit with 4xstgs to only repairing?
Also, I would never get the shrek upgrade, EVER. Its a 4 man squad with a shrek and they feel squishier than a volks squad whilst being more expensive. After ~10 mins, they struggle to hold their own against ALL allied infantry, so why give them a pzshrek, when the squad is likely to get 1-shot by stray tank shells whilst helping repel an armoured assault? That would also mean you need 2x Sturmpionier which is a terrible idea as you need a minesweeper always.
Not sure where is the comparison with other factions engineers. I just showed you how vet works. EFA engineers have way lower than normal combat bonuses. RE/RET have slight below and different than normal vet accounting that they can equip either AI or AT weapons.
In case you miss it, they have the same vet bonuses as other combat oriented units and +10% over CD values (5% if you take into account automatic weapon users).
I'll give you an advice word: only a
sith noob deals in absolutes. 95% of the time you'll want a sweeper cause you are more than likely gonna have a singular SP. Apart from playing 1v1, on teamgames you can depend on your teammate and your options increase. Depending on map and match up, you might find yourself with a double OKW, going double SP or just facing USF (which tends to lay way less mines and besides M20 they are less dangerous). When game drags to mid-late game artillery can be a factor so mines get blow up as well.
The upgrade is not there for you to spam SP and chase around tanks but may them think twice when they try to dive your line to kill your Schewerer or raks.
Just because you have it, doesn't mean you need it. Just because you don't always need it, don't forget it's there.