I'm playing with the idea of combining an Ersatz Eye (XGtE, p137) with a spell focus. Since the Eye is stated as being irremovable "...by anyone other than you...", does it make the item unbalanced?
This is specifically in regards to a sorcerer, but I suppose it applies to any caster that can use a focus. It seems like it would generally be no big deal, but it ultimately means that the character could technically never be disarmed.
I've played a paladin whose focus was tattooed on his face for precisely this reason. At my table, component tracking isn't something we've really focused on, so we haven't seen any balance issues. However, if your table tracks components more closely it may be less balanced.
There are also relatively few instances where the material component becomes a hindrance. If your character is wielding a sword/board combo, then this can be more of an issue. My paladin was a heavy weapon specialist and the rules do allow for a hand to come off the weapon which minimizes the mechanical requirements there.
But a character whose hands are each occupied with two different things may tip the balance of this. If not, then it's of minimal impact from my experience.
If your DM allows the Disarm optional rule or if they allow a mechanic for taking items from other characters there is more of a concern here - but that's easily circumvented by multiple pouches/foci.
One downside to my decision to tattoo myself and mark my armor is that I'm quite recognizable. At times it's been a benefit, at others we've had to figure out a way to conceal me.
Since attunements are quite limited, that is a significant price to pay to make a focus non-removable. (You could use the Ersatz Eye as a focus without attuning it, but then you would not be able to see out of it, and it would still be removable, as those are its magic features.)
I had a warlock in my game, that I DM, who had his arcane focus be a gem stone that was inset into their forhead by their patron. I breifly thought about whether or not this was an issue for me/had the player taken it to far. I decided it wasn't.
In-game there has never been a time (so far) where someone has been disarmed of their arcane focus, that doesn't mean there wouldn't be. In theory my player received a greater benefit but in my opinion it was a small one.
I don't believe it unbalanced the game at all, at any point, and that character lasted a good few months. In all the games I have played in the arcane focus is very rarely roleplayed out as well as it should be anyway.
