[Schwag]

I have sold upwards of 30 of these things without a complaint. Quite the opposite. They get great reviews.
I own both a jg mp5 ras metal body and a echo1 mp5k.
Out of the box, they both ran great. The mp5k has a shitty battery that dies very fast and the chargers are shit. The jg battery is great. The gun is almost 400fps and the mini battery runs it all day.

I have heard of some almost doa guns but no more than I have heard coming form CA or Marui.

Upon teardown, there are differences in the quality of some parts (piston+head, cylinder, hopup.)

After changing out the above mentioned parts in my JG and adding bushings and a reshim, the gun now does around 400 fps. Up from 360-370 stock.
No spring change.

They do sometimes have gobs of cheap grease in them that needs cleaned out.

My echo was not as nice as my JG. It needed a reshim, cleaning, and a new piston head. And the battery sucks hard.

I paid $175 for my jg and have put about $100 worth of parts into it. Not for power, just durability. Have gamed it maybe 10 times and I love it. I use it over my marui.

The mp5k runs fast and hard but has NO accuracy at range. Installing a tightbore to compensate.

If you buy a stock marui mp5 for $450, you still have to install a better guide, spring, piston, tightbore, bushings etc before you're shooting a reliable 400fps and that brings it to around $600 even if you're not paying tax. AND you still have to buy batteries

I think you'd have to be a serious freak for trades or have a love for burning cash to buy a name brand gun anymore.
As for ptws. I don't get it. I have yet to see one give anybody a clear advantage and they cost a fortune, break constantly and only come in armalite(for now)