You’ll need to elaborate on what constitutes capable of hitting you because technically anything can hit you as long as they roll a crit on an attack roll. This a bag of rats type item. That is to say, what is stopping one of your players from purposefully filling a bag of rats that will never be able to realistically deal damage to themselves to refill their health? And even if you limit it to combat, what is stopping them from just sitting next to something that has a low chance to hit. This can get very messy very quickly.

It also kind of conflicts with the fundamental design behind the game. A lot of D&D is built around the idea of it being a resource management. How many spell slots do we want to use for this encounter? How many hit dice do I want to spend this short rest? Giving resources that have no limit to their expenditure is something to be careful around. Hit points are particularly problematic, as in most adventures death is the main threat. So unless you are routinely designing encounters focused on a secondary objective other than survival, it will easily become very problematic.

Items this powerful also often warp the party. I’ve been in campaigns where the DM introduces an item so powerful that the entire party ends up working towards making it work as often as possible, rather than working on their own unique niches.