Strengths and weaknesses depend on builds, but they tend to be better at different things.

Engineer CAN be the best at anything short of killing really big bugs (drillers are unmatched in that department) depending on the build. They have a lot of viable builds and can be the best at AOE with the PGL nuke, best at single target with Hpropellant PGL or Breach cutter and still have the sentries for AOE. The engineer hunkers down, fortifies a position and kills the wave before it gets to him. If he can't outdps the swarm he is in trouble, he does not fight well on the backfoot and is the weakest  of the classes in a scramble (chaotic, everyone is kiting) situation.

Gunner is good at single target damage, but relies on bullets. So even with a build tailored to kill big bugs he tends to be the weakest at killing larger glyphids, lacking the required elemental effects and burst damage to get the job done. He is also the slowest class, but benefits from the shield generator, which makes him tie for the strongest clutching class with scout (scout is better at kiting and escaping, but can't ress swarmed teammates or deal with waves on his own unlike the gunner). Gunners AOE is only second to the engineer with the autocannon, which has excellent ammo effeciency. Gunner is unique in that he doesn't struggle in close quarters like the other classes.

The impact of each class in fights:

Engi&gt;Gunner≥Driller&gt;&gt;&gt;&gt;Scout

In a group of equal skill, the engineer should always be doing the most damage, Gunner and driller are interchangeable in practice, but autocannon gunners have an edge over a driller. 