Not my experience of it . The CO2 box mag is very good , I find I go through much more ammo than with the green gas mag as longer bursts are not a problem for it and the 249 properly rocks along on CO2.
That said, there are a couple of issues that we have come across. First the size capsules the mag is designed to use are not easily available in the UK, most of us are running 88g capsules, but that does require modding the mag as they are longer.
We have also had a couple of occurances of the hose failing, although it's any easy fix. Have a read through the VFC M249 thread in the gas section.
They can be converted to hpa and then use an aeg mag, but for most of us that spoiles a big part of going GBBR, mag changes , limited ammo , not looking like a ghostbuster , if you want to spray and pray go hpa. Horses for courses , but the CO2 mag is worthwhile for winter use. I used the Green gas one all summer without any issues .