Hello!
The USB problems seemed to be mostly related to memory drives, not peripherals. My mouse has been plugged into the left side and never had a problem. As I recall one of the BIOS updates addressed said USB issues to some degree. The random restart thing is a very rare case. Only such issues I've found are related to using ASUS GPUTweak and Intel XTU. There are no manufacturing faults to speak of, I've seen 5 G751s and they were practically identical in every single way, they're built very well in my experience. It feels reasonably stable. Audio disbalance issues were resolved by means of software, some people still report crackling issues as can be see in one of the threads here. Some users report backlight bleed on the screen but this is difficult to spot unless the screen is a dark color.
Regarding the bay - if you buy a model with just a hard drive, no SSD, you will get the hard drive in its own caddy and another 2.5 inch caddy preinstalled for you where the M.2 bay is. If you purchase one with a M.2 preinstalled, iirc, you get the M.2 bay holder, but no 2.5 inch drive caddy to use if you want to use that type of drive.
Be aware that some of the laptops that come with just the HDD don't have the M.2 socket added - just the bare connectors on the board. Mine was like this but after I RMA'd it, they changed the board with one that has said connector.
What exact model are you looking to buy? There are some model - specific quirks that I can think of that a lot of experienced users here are familiar with. Depending on how much memory you have preinstalled, there are some things to note. First, you need 1.35v DDR3L memory running at (recommended) 1600MHz. Second, whatever memory you get it with, up to 16GB, will be installed under the keyboard and cannot be user accessed. If you get it with more, the 2 slots under the laptop will also be filled. I personally advise getting one with 16GB of memory, it's quite sufficient, I make do just fine with 8. If you have your own 2.5 inch SSD and it's of high class, I recommend staying off the M.2 options but if you get a good price on one, it might be worth it to go for that.
Overall, the G751 is great value and very reliable in my experience. The screen is amazing, the keyboard feels amazing, the sound is good if you know how to tune it and there's nothing on the market right now that it won't run well at High settings (some games like Witcher 3 give it a hard time).
EDIT: Forgot to mention that a few months ago some users were getting laptops that had some of the keys around Tab, Tilde, ESC etc. be tilted in and looking broken. Not seen this personally.
