Ender 3 cracking magnetic bed
My personal technique for removing prints (particularly larger prints) is not to flex the mat at all. Instead, I wait until it's cool and then work the spatula under an edge and move it under the model to basically peel it from the mat.

Smaller prints usually pop right off if I start with this technique from one end. 

Surface area on the bed (for a single part) is most important to how much work is involved. For example, a raft or brim will greatly increase the surface area.  The spatula is long enough that usually I can reach everywhere under the model so eventually it will pop free. I might have to work around the model a bit before it reaches this point.