Also, as this causes to halven the shutter speed, it's not an option when using a tele.
Not sure why it isn't an option with a tele?
Eeerrr... in order to get sharp images?
Unless you're using a tripod or can arrange a very comfortable shot otherwise.
Ah - you mean at low shutter speed?
Depends on how low is low for you
Honestly, when shooting at - let's say - 300mm, I consider 1/800s to be the threshold for APS-C and 1/500s for FF.
Stabilization and individual steadyness can do a lot about it, but I never trust them too much.