Having such a hard augmentation limit feels really strange to me when augmentations both have level-based effects and some augmentations are clearly meant more for flavor than mechanical power, or otherwise simply don't scale as well because the effects are level based.
Having a more freeform limit than slot-based is nice, but the limit is rather small at early and mid levels for characters not heavily investing in Con, so I think there needs to be some concession for cosmetic augmentations or lower leveled augmentations that may be less powerful than their more advanced counterparts.
Beyond that I'm a bit iffy on the limit in general. Like I said, it makes some sense mechanically and conceptually to have some limitation, and tying it to physical toughness (i.e. Con) makes sense too.
But building a frail character is already generally a bad idea, denying them equipment slots on top of that feels unnecessary at first glance.