There's the added factor of operating your aircraft at a more nearly constant thrust-to-weight ratio. What does that mean?
It's pucker factor. If you typically operate from long runways, at moderate TOGW, at moderate density altitudes, and use rated thrust all the time, you get spoiled.
But then comes that long leg out of MEX. The book says you are fine, but the plane seems so doggy and you've never used so damn much r/w before and HOLY **** WILL WE EVER MAKE VR????!!!
OTOH If you're used to flex takeoffs, that MEX takeoff (while within AFM limitations) won't seem so terrifying.