Engiebots are super easy to counter

His engiebot AI forces him to "retreat to cover" for a few seconds if his sentry is destroyed. It happens to be super easy to destroy his sentry when it's constructing, so you can easily force him to be useless for a few seconds by strategically shooting it down early. On the other hand, engiebots' teleporters have 300hp (double health), they immediately start with full health when they are constructed (unlike regular teleporters), and as if that weren't enough, they also continuously regen health as they construct. So it's usually best to delay take out the tele until after destroying the sentry, since the sentry is much easier to kill, and it has the benefit of making the engie wander off uselessly for a moment.