Wikipedia breaks down combat engineering into a handful of disciplines:
Mobility (clearing obstacles and building roads & bridges)
Countermobility (creating obstacles, sapping roads & bridges, planting mines)
Handling explosives (demolition, and clearing mines)
Building defense structures (fences & fortifications)
These seem well covered by skills:
Mobility: Civil Engineer
Countermobility: Civil Engineer, Explosives
Handling Explosives: Explosives
Building defense structures: Civil Engineer
I still don't feel like it warrants its own skill.
If you're trusting Wikipedia, well, that's mistake #1.
I'll note, in counterpoint:
Demolitions work is a separate MOS from both Civil Engineer (Enlisted CEN aka constructionman) and from Combat Engineer (CBE). Dem Techs are assigned to both types of units, but are a separate specialty.
The Army, which actually uses them, keeps them as separate training regimes. Not just in the Army, either. The USMC does, as well.
When you need a bridge tomorrow, you call the Combat Engineers. When you need it to last more than a month, you call the civil engineer, who has an officer work with a draftsman to design it, and then the CEN enlisted build it, using different construction techniques from the CBE.
The vehicles used are also different. Civil Engineer asstes include the usual earthmovers. Combat Engineers use modified APCs, sometimes with dozer blades, sometimes with chain-beaters, and self-mobile bridges, and lots of shovel, axe, and chainsaw.
Just as you don't want the Civil Engineer/architect designing your automobile, you don't want the mechanical engineer designing your bridge. He lacks the correct knowledge base to do the job at lowest cost.
And the combat engineer isn't trained to design things at all. He's trained to jury-rig.