The appeal of resin to me is that very small runs of unusual items are available. As an example, I'm planning to build an early M 48 model just now. The kit will be issued by Dragon, who no doubt will cast many thousands of copies.
But their kit will be of the A1, which were numerous, rather than the M 48 "no letter" which was relatively rare. I want a 48 nothing, so I have looked online and found the conversion parts I will need, which are probably made to order or in small shelf stock.