The Engineer can only train up in pistol skills, so he won't be able to use assault rifles, shotguns or sniper rifles. Even with this limitation the pistol can still take down several enemies, as long as you use its powerful ability called Marksman which allows you to fire more quickly and accurately. When you improve these two stats, you can take enemies down faster. The pistols won't have the same firepower as sniper rifles or shotguns, but if you are a skilled marksman, then the pistols can handle most of the opponents the game tosses at you.
The key to combat for the Engineer is to use his tech talents to set up scenarios that can protect and support his squad, like destroying enemy shields and shutting down enemy fire, creating opportunities where you can attack more aggressively at the same time as escaping from massive hits.