Don't under estimate the power of re-rolls. There is a reason Advantage isn't dolled out buck the bucket. Negating a Fumble and having the chnace to turn that into a hit, or even a crit IS pretty powerful. At Most I'd give it at up to 3 uses per short rest (encounter).Thank you for the very detailed feedback!
A lot of people don't like the 1 automatically becoming a 10. I think I'll follow your advice and allow the player to reroll a 1 instead and must take the new roll, even if it's another 1. I don't like restricting it to once per short / long rest, as I don't think that's necessary. It's not that strong.
As someone with actual weapon training,weapons such as mace and hammer are considered anti-armor weapons primarily for two purposes. Infantry use them primarily to damage armor's soft points. A good fighter can take repeated blunt hits to the strong points of the armor without flinching, but if you can damage the soft points, you severely cut down their ability to fight effectively and make them more vulnerable to other attacks. Cavalry use blunt weapons for primarily one purpose - head shots. Combining the mass of the weapon with the velocity of the horse is usually enough to smash in many helms, or at least enough to make the helm a liability due to restricted movement or line of sight.When modelling the specific bonus of a mace, I did some googling. In most of the sources I read (admittedly they were all online and not particularly in depth), maces were in medieval times "armor fighting weapons". I wanted them to extra effective against armored opponents. I agree that the most elegant way of going about this would be to make a single feat for all bludgeoning weapons. I just like there to be more granularity and differentation between the weapons, not just the weapon types. With that said, I think the feat you suggested, giving unconditional stun for 1 round on a critical (no saving throw) is a bit too powerful. Against bosses, because there is no saving throw, the potential of just losing 1 round would be catastrophic. The variance would be a bit too much, I think.
As for game mechanics, look through the PHB. There are a lot of abilities that Stun for 1 round no save. Most Stun attacks that require a save effect the target for longer than 1 round. Additionally, it only stuns on a Crit which is another major balance point.
Feats just aren't meant to be so highly specialized as to effect only a certain few creatures, which is what I was getting at. The Mage Slayer isn't really specifically about mages, its about any spellcasting target. If you want a feat you can call an Undead Slayer, it has to have abilities that are aplicable to a wider range. Having an ability that specifically effect only 3 types of undead, to me, is too specific. An ability like Advantage vs Paralyzation, while not technically undead specific, is still not generally useful enough to beconsidered a feat ability IMHO. Something like Advantage on CON saves is much more useful but needs to be limited either to something like "from undead" or in uses per short/long rest. JMHO.Simply put I don't want to make a feat that only work if you meet a particular type of enemy. I want to make a thematic feat, based on a type of enemy and several of the most iconic creatures in that type and / or scenarios that this type of creature evokes. The first bonus comes from the iconic ability of ghouls (but also liches and revenants). The second bonus harkens to the iconic imagery of being surrounded by hordes of ravenous dead (zombies, skeletons, etc. often attack in large groups). The third bonus shuts down the zombies Undead Fortitude. If all the bonuses from a feat only work against a specific type of enemy, the feat is far too narrow.
That said, feats are the most difficult part of 5E to finesse out the design principles. WotC really needs to provide a better set of guidelines for their design IMHO.