Safety depends upon margins doesn't it.
Let's say you've got a typical light aircraft, MTOW around 2000lb, TODR around 600m, stall speed of 50kn, g-limit at 3.8g, Va of 97kn, initial climb rate around 500fpm at MTOW.
Add 10% weight (to 2,200lbf) and a back-of-envelope sum tells me that, you now have a TODR of 720m, a g-limit of 3.45, a stall speed which has increased by about 3 knots (and so have the initial climb and approach speeds), and a climb rate now of 450fpm.
All of this has to be accounted for,....
G