Plastic anything. Low quality screws getting stripped. Flimsy wiring. (I'm not the OP, but I'm speaking from my experience.)

In the old days the plastic bits were often sturdier and held together by long screws. You had about a 75% chance of getting them all out without stripping them, if you had a long skinny screwdriver.

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.

How can you spot low quality screws? I thought you'd have to stress test them.

What do you mean by flimsy wiring?

I design a lot of consumer electronics. You obviously notice the way they strip obviously, but you can run them against a file to see how hard they are if you’re trying to choose parts/suppliers. There are other things to look for, like how precise the slot is or how smooth the outer edges are, but that’s not as important. IMO stress (shear, stretchy) tests are almost totally irrelevant for consumer products that have plastic or soft metal enclosures which will deform far before a fastener will break.

Let's say you are given a product, which you can disassemble as you like. can you give a reasonable estimate of lifetime this way ? or does it require prolonged stress tests ?

And if estimation is possible, how does one learn how to do it on their own ?

This is more of an experience thing, but it is usually very obvious if something is poor quality. A good thing to look at is typically the plastic parts. Look for signs the mold was in poor condition or cheap such as excess flashing or surface roughness. Another thing to examine is how thin the parts are, and presence or lack of reinforcing ribs. For comparison, take apart something like a Milwaukee cordless tool vs. a cheap no-name equivalent. AvE on YouTube does these kinds of teardowns, and does a decent job of explaining design decisions, but is definitely NSFW.

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).

+1 for AvE[1]. He's NSFW compared to bible school due to cussing and occasionally dirty jokes, but he uses language that pretty much any manual laborer in the country encounters daily. His analysis of how things are designed have taught me all sorts of things about how tools are made.

[1]https://www.youtube.com/user/arduinoversusevil

I would also recommend this guy's youtube channel, he disassembles and examines both normally priced and suspiciously-cheap-from-china consumer electronics:

https://www.youtube.com/user/bigclivedotcom/videos

cheap consumer electronics can be assembled in a method with screws intended to be used once into a plastic cylindrical receiving part (injection molded usually). The screws have a certain sort of spiral to them that you don't see with screws intended to be repeatedly installed and removed. On the cheapest stuff, removing the screw once and then reinstalling it can result in the clamshell assembly of the upper and lower injection molded parts becoming permanently loose... One example would be a "modern" humidifier such as you can buy at Target that is never intended to be repaired or disassembled.

Those are self-tapping screws.

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.

Those all seem like things you could fix

Yes, but there's a certain futility in spending 30 minutes repairing a $5 widget when the $8 widget wouldn't have broken in the first place.

sometimes there isn't enough plastic. but for slightly higher quality goods the threaded standoffs will often have enough material to take a wider machine thread, and enough strength to keep them for several cycles.