My first build is the 49cc flying horse with the NT carb- yes, the rear engine mount was small for the bike, the front mount close to useless and the magneto needed a bit of wire work... I also had to enlarge the sprocket hole to mount it... and eventually replaced the ragmount with first an MM mount, then the Howard mount (which I prefer)-built it back in 2010. Just had it out today for a good romp. I am not dissapointed by it at all.
None of these kits are 100% fool-proof... some are bit more reliable than others, but even the Grubee can have quality control issues. I am not familiar with the carbs that are currently showing up with this year's stock of Grubee motors, but the ones I got last year took a bit of effort before I got them to do right - they're CNS operations- but once I figured them out, they do the bat-out-of-place-of-eternal-punishment routine. Someday, I will slap my NT carb on a Grubee and see if the carb is the difference or the motor