Ah, but the trouble is it’s not immediately obvious when they’re going flakey, which the cheap ones will do quite readily.
A more grievous sin is of course abusing ‘K’ connectors. These mate with SMA but work to 40 GHz. 
And actually ebay tat doesn’t save you that much over proper connectors from a proper supplier. A decent crimp plug’s about 4 or 5 quid IIRC.