Sometimes clones leave off certain things that are necessary, an example is the EHpro GGTS clone, they left the oring off the air flow control thus making it non functional unless you find the proper size oring to fit it, also the 510 extension is fixed so so some devices will not sit flush with the top.
On the other hand the Kay fun clone from EHpro is identical.
So do your research as to which clones are good and which have issues.
here are some makes of clones that I have experience with and my opinions.
Rainbow Heaven- great clones even to the point of parts being interchangeable with the real thing
Tobeco- hit or miss, the seem to have improved, but most will remember them for the debacle they made with the Oddy clone (or Oddity as it has been called)
Sigelei- Eh what can I say they seem to think an ego connection should be on most mods
I have other clones but dont know who made them (one is a Kayfun which in a attempt to make a replica they even copied the "designed in Russia Made in Germany" and logo on the bottom, then stuck it in a black box labeled "Made in China")