I would not expect one of these to fall into a PC's hands without them winning some major battle - the cultists who carry these are high ranking. The DM who lets a PC just grab one deserves the unbalancing effect :)
Used without the counter-balance of a pyschotic, large, magically powerful and infernally well connected cult, and yes it would be unbalanced. They do scar the character in question - I left that to the GM to adjudicate, but I would be looking at both charisma and sanity impacts - they are neither aesthetically concerned, nor do they anaesthetize while operating. Not really that much different from old style medical practice when you think about it :)
They cannot be used in the heat of battle - both practically and with the chance the injured party could end up undead should the patient die mid-operation.
Additional complications one could apply:
1. The food on hand must be a live, aware creature of some form.
2. The hole in reality the amulet opens gives a possibility something else could step into our realm.
3. Gyatharmar may notice and either act himself (unlikely) or at least communicate the location to the Cult.
4. If the lesser bonestitcher is damaged during the operation (i.e., it was called upon during an active battle) that particular one will never return, removing one of the daily usage slots.
5. Each successive use on a given day adds 1 to the roll on the table.