that skill is just like ♥♥♥♥, the only playable part is the first aid tent, the ballista is worse than just 2 mortars even after you upgraded this level to level 4. It don't deal enough damage for a level 4 skill and its trajectory sucks which makes it misses a lot resulting its actual damage even worse.
And as for the faction-special equipments for engineering, the usable ones are:
1.Mimicry Pod: summons shadowclones of the enemies;
2.Thorn Idol: summons thorn giants;
3.Venom Sprayer: send down toxic blobs just like Toxicology, so it doesn't have a "trajectory".
As for the rest? The rest is just some enhanced "ballista" with the same problems of it.

When it comes to the subskills, planning is good but not for your first hero. Sapper? not that good, I'm afraid. The mason only works when defending. So, when there are other builds that can easily slaughter those ai, why would i pick this skill?