MP5s are a pain in the ass to disassemble compared to other airsoft guns. The CA metal upper is of the "easy disassembly" variety ( taking from their Armalite series) and makes it a bit easier.
From what I've read, the metal series of JG MP5s seem to have a high problem rate compared to the ABS models. They have chronic issues with being stupidly overpowered (400+ fps) and cracking mechboxes and exploding internals (due to the power), and having off-spec receiver parts, etc. That's why I went the ABS model route instead, and glad I did.