Joining a KMC chain, wrong quick link, wrong technique?

[mattr]

you can just join the chain using a chain tool

No, you can't.

Many modern chains (basically since 8 speed) have peened over or bulged out ends on the pins. As soon as you push the pin out, you will either lose the ring of peened material, or stretch the hole in the side plate.
This is why shimano have an over size joining pin (to account for the stretch/larger hole in the side plate) and everyone else uses quick links.

Doing like you used to in the 80s is quite high risk.

 

[Dead Rats]

^ amen to that, I snapped a 9s chain uphill once after I fitted the chain without a Powerlink and needed to use it! Still got a bump on my rib from the ensuing (slow) crash!

 

[mattrixdesign2]

I have joined a chain in an emergency using a tool, but prefer to use the shimano pin on shimano chains. Used a SRAM quick link before and it was reliable, have also used a KMC link and it failed badly next to the link (a weak point?).