GBBRS are reliable depending on which brand you buy obviously, Most GBBRS are all based off the WE, or WA platform, Super high enders like Innokatsu are amazing, but replacement parts are super hard to find, as there arent that many after market parts for them, WA wise i would get the G&P WOC GBBRs those are FULL METAL and pretty much the same price as the original WA ones but the WA ones are ABS receivers so would probably stick to the WOC or WE guns.
I run a G&P WOC M4A1 which i have completely Pimped out
I would never get a ACM GBBR as ive seen them completely Fudge up hard on the field, For example, my friend in Hong Kong, his ACM M4 GBBR, the Charging handle completely snaped due to its brittle crappy POT metal that was used to make it and that was just bad, its now a wall hanger in his apartment =.= ACM guns maybe based off the WA platform? but wouldnt go for them
In terms of KJW, heard both positive and negative comments about it.
But as most will agree with me, stick with WE or the G&P WOC if u got more cash
The VFC and SOCOM GEAR M4, i have no idea as i have never played with one before but have heard many positive reviews about them.