Enchant restrictions are wrong.

Stats on an existing instance of a weapon are fixed and don't change with updates (like older ranged weapons missing piercing), but enchantability is checked through tags at the time it's attempted, and tags are based on the item class (so they do update themselves now and then). I checked to be sure, and it looks like they removed/lifted/changed the restrictions on all the guitar models.

Good to know, thank you for investigating, and that insight about item checks during enchanting.