Plastic anything. Low quality screws getting stripped. Flimsy wiring. (I'm not the OP, but I'm speaking from my experience.)
Thirty more years of practice with injection molding and the addition of better stabilizers means the pieces are thinner.
But sometimes they glue the pieces together. Or they use snaps to simplify the assembly line and reduce the plastic further. They squirt in a bead of silicon to prevent vibration or shorting damage.
And part of me suspect that in The Old Days they used to repair products at the factory. Manufacturing processed have changed substantially starting in the late 70’s to favor more and more detection of problems during the assembly process (see Toyota).
If the defect rate is high you repair things to keep your boss’s boss happy. If defects drop and manual labor prices go up then it’s less of a payoff. Or none at all.
What do you mean by flimsy wiring?
And if estimation is possible, how does one learn how to do it on their own ?
Stress testing screws would be pointless for what you want to achieve, but like the other commenter said you can look at them too find signs they are poor quality (irregular heads, bad plating, etc).
In the old days, items of wood would suffer from the same issue when disassembled, although it was then much easier to fill the hole with putty and redrill.
With plastic, I suppose you could use epoxy to fill the hole.