I have bought a lot of cheap components on eBay from China & Hong Kong. I'd say about 15% of the time the parts are fine, another 75% work, but not quite exactly as per the data sheet and / or have been poorly manufactured in some way. The remaining 10% either don't work at all or fail very soon after installation.
So the bulk of the stuff ( 75% ) isn't quite up to par, but in many cases still useful, particularly in undemanding applications ( a lot of hobby circuits could be said to fall into that category ) However given that the majority ( 75% ) of the parts fall short in some way it is definitely NOT a good option whenever close spec or high reliability is called for
You get to learn which sellers sell good versions of certain parts with experience. With eBay there's always the option to return for a refund. With some potentially good, even great bargains to be had it can be a lottery worth playing.
I often wondered why the Soviets chose to make the Lada cars ( one of their chief exports to the west ) to such a poor standard, was it a covert form of warfare! They were capable of building good stuff. I've bought quite a lot of Soviet surplus components on eBay and they've almost all been very well manufactured, so it seems bizarre that they would choose to showcase an inferior product to the outside world!
Another manufacturer of poor electrolytic capacitors is CapXon they are used in low cost consumer products and are frequently the cause of equipment failure.