As I mentioned in my look at the Haley Strategic Flatpack, the best usage case for it is probably when combined with a chest rig and the best version is the grey one since it has the spacer mesh to go up against the wearer’s back, rather than just a load of PALS webbing.
The key thing I need to point out with any setup like this is that they can be a nightmare to balance. If you’ve never tried one it might not strike you at first, but when the straps at your sides aren’t at least somewhat rigid to keep the front and back in line you get seesaw issues depending on the weight you put in either the chest rig or the pack. If you’re only carrying super light stuff you can just tighten the straps a little and you’ll be ok, but otherwise you will want to consider setups other than what you see here.
If you want to put any more than about 0.6kg/1.2lbs (very rough ballpark) in a chest rig or pack, look at just wearing the harness for the chest rig then putting on the backpack with its’ usual shoulder straps. It takes a little getting used to managing the spaghetti and even more so once you add in a sling, but with a little practice you’ll get used to donning and doffing the whole ensemble without getting tied in knots. Otherwise, even if you perfectly balance out the equipment loads front to back, the thing is going to dangle one side or the other as you shoot/drink/eat/throw ordnance or whatever the heck it is you personally are doing with your gear.