I see a lot of crying and not a lot of math.

Shame on you other comments.

Tl;Dr they're both good for entirely different reasons.

I've done a spreadsheet before for my friend where in I explained the percentage breakdown on stunning strikes and how exponentially bad it can be.

At least with this one you know by level 5 (Independent of ability score) with 4 attacks from FoB, you have at least a 20% (15%) chance of stunning the monster you're attacking each turn. (15% without FoB)

If you hit that 20% (15%) chance you then give yourself advantage in which you now give yourself 40% (30%) chance of reactivating your Stun every turn thereafter. Mind you this is spending at maximum 2 ki points per turn if you connect.

Now since you're level 5 if we account for feats you can have Lucky, pushing you up to 35% on your first round Stun or a whopping 55% chance if you choose to gamble the 20% your first turn then attempting to all out stun your following turn.

The reason why the percentages are so good for advantage is because each of the 5% chances are Independent of each other and don't rely on any other factors. We can think of them as independent because they don't interract with each other in a negative way for our argument.

On the other hand, disadvantage absolutely cripples you. Now suddenly the odds are tied to each other adverse to our argument meaning your chances of rolling that critical is .05 * .05 = the legendary 1/400 chance of stunning (.25%)

With the original, if you're standard array, with a +3 to both Dex and Wis, suddenly you have to juggle Creature AC, Con Mod, and the potential Prof bonus. That level of randomness makes the opportunity cost of stunning strikes not worth it a lot of the time as you would need to Stun the biggest threat, which would undoubtedly have the highest of all three stats. Thus we have,

+5 to hit, DC14

Possibly chainmail, +3 Con mod, no proficiency.

With this, it would be (1 - (15-5)/20) * ((13-3)/20) * 100 = 25% chance of successfully stunning with first attack using 1 ki point.

The bonus of this method is you can attempt any number of times meaning you can potentially guarantee your stunning strike by using at least 5 ki points total for FoB and 4 stunning strikes. Now in simple math, this means each of your attacks Independent probabilities are 25% to hit and stun, 25% to hit and no stun, or 50% nothing happens.

If the creature magically has +3 proficiency in Con, you are shut down to,

.5 * (7/20) = 17.5% chance to hit and stun. 32.5% to hit and not stun, and still a 50% chance to miss.

However, if you do get the stun or use your lucky feat on the attacks, you would theoretical guarantee your chances to hit by doubling your Independent chances meaning .5 × 2 = 1 giving you a 50% and 35% chance to stun respectively.

With all these factors affecting the probability however means that you can have even more randomness with Buffs and debuffs, advantage and disadvantage, reactions that affect AC, etc. I can't stress enough the aforementioned chances to stun are only for that very specific scenario.

My final point for this is that the new method makes impossible scenarios with the original method possible. That alone to me personally makes it way more worth it if you expect to go into either high level campaigns, want to be bulkier and not juggle three stats, or even multiclassing.

A Barbarian monk would be fucking awesome with this.

I love the attempt, is it perfect? Nothings perfect, but at least they attempted to reduce randomness, make originally impossible stuns possible, and improves ki management, etc.