The problem is that nowadays very many are fake, you can't tell without buying, and of course the fakes
undercut the genuine very rapidly in the market without a way to get back at the criminals
involved. Best to only buy from reputable suppliers who'll try to avoid the fakes and reimburse
you if any slip through.
Many fakes sort of work, but are cheaper less performing devices re-marked as the more expensive ones,
or simply testing rejects (which are supposed to be destroyed) that someone's then packaged anyway.
for instance its a pretty common experience on the diyAudio formums that buying a 200V 16A
audio output transistor gets you something more like a cheap 60V 4A part in a faked package.
Consequence: they tend to explode in circuit.