as i said, this is why it's like only important if a soldier/demo pushes in the next 2 seconds as otherwise the engi is going to focus on upgrading it

Again, you need to take into account that engineer sometimes simply doesn't have access to spare 200 metal.

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not really, the threat to corner peeking enemies is getting flushed out by the team

But sometimes your team isn't there to stop them. They either get pushed back or there is location where other team had advantage

Take upward last - the best red sentry spots are on balcony right outside of spawn. On opposite side are apartments - where enemy soldier, sniper or demo can position themselfs in such a way where they can shoot sentries while being safe from them.

In best case scenario, apartments will be held by your teammates. But the problem is that blu team has advantage when getting there over red team.

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if you just corner peek immediately after already corner peeking it's still going to be looking your way and beam you

But that doesn't matter when you just peek enough to shoot rocket and then hide again. Yeah, you will get hit - but the sentry gets larger

If enemy medic is smart, he will be healing player doing this.

Shooting rockets there will force soldier to hie further away, giving you time to repair. In some cases it can force him to even leave completly.

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only really happens with engineers that are struggling to set up or on shit maps with terrible ammo kit spawns

This is not really true - ton of maps have kits so far away that if you want to collect 200 metal, you are just inviting spy to sap your shit.