I've mained mnk since I started playing in 2.4, and I have to say that I'm frustrated and disappointed with the design decisions that seem to have been made regarding how mnk is going to function in Stormblood.
The Tackle Mastery system seems clunky and too situational to see wide use - we're still going to be in Fists of Fire 99% of the time, and while having an extra Shoulder Tackle is nice, the two from Riddle of Wind put together have the same base potency (130) as the standard, no-frills version, but without the benefit of the flat 5% damage increase from FoF. I also question the usefulness of Riddle of Earth, considering that the proc apparently requires you to take damage...in order to take less damage and potentially trigger the GL duration increase (I don't even know what's up with the latter). In endgame content (or in any content, honestly), you do not want to take avoidable damage, let alone deliberately seek it out to activate an effect with dubious benefits. I also don't see the benefit of dropping Tornado Kick's cd to 10 seconds. It's a situational ability to begin with in that you only pop it if you know you're about to lose stacks or the boss is about to enter an invuln phase, and since the GL refresh from RoE potentially mitigates both of those scenarios to an extent (if you can, you know, actually manage to get it to work), drastically lowering the cooldown time on an ability that we barely use to begin with leaves me baffled as to how that might ever be advantageous.
We're also losing essentially mandatory (for endgame at least) abilities in the name of minimizing button bloat (B4B, Fracture, ToD, Mercy Stroke), and as a replacement, we get...a trait and set of abilities that we're apparently supposed to keybind all three Fist stances and all three Riddle abilities for, two and a half of which seem dubiously useful (30% damage boost on Riddle of Fire is lovely, but having the 15% gcd slowdown negates GL3 - which is one of the core mechanics of mnk as a class). What we're receiving in exchange for some of our dps (and the ability to hit ToD/Fracture to avoid clipping Demo and squeezing an extra tick out) does not seem to compensate for what is being removed, and if it does, it's entirely unclear how.
Also the loss of ToD and Fracture renders our core rotation simple to the point of boredom. DK/TS/Snap. Boot/True/Demo. Repeat ad nauseam.
As for Brotherhood? 5% increased damage for 15 seconds is great. 5% increased damage for 15 seconds on a 90-second cd that doesn't apply to the person using it is...less than great. The chakra gain from it depends on rng twice over; if your group is primarily casters, you start out at a significant disadvantage since the proc is based on weaponskill use specifically, and the proc itself is a 30% chance.
Forbidden Chakra is still (as far as I'm aware) on a 5-second cooldown, and the animation lock is long enough that you can't realistically double-weave it, which seems to be what we're apparently intended to do with RoF's gcd slowdown. Both Brotherhood and RoF have a 15-second duration.
If rng is in your favor, the maximum amount of boosted Forbidden Chakras you'd be able to get in either window is 3, and that is with FC nerfed to 250 potency.
Our burst capability will be based on rng, and ultimately isn't much of a burst at all, especially after taking a 10-potency nerf on our entire core rotation and dropping Demolish's duration to 18 seconds down from 21.
With all of these facts taken into consideration, the limited raid utility from Brotherhood and role actions (that we share with the three other melee dps classes) is not enough to render mnk useful or viable in comparison to drg and nin's personal dps and additional raid synergy/utility, and especially not in comparison to sam, who is looking to fill our role as The Selfish DPS (TM) better than we ever did.
I'm lucky that my static is willing to keep me, but I'll be leveling sam regardless, because as it stands I'm looking to be dead weight on the class I enjoy most.
It's sort of additional salt in the wound considering that Stormblood was pitched as The Monk Expansion (TM).
Tl;dr - The proposed changes to mnk are dubiously beneficial at best and actively detrimental at worst, we get nothing truly new or significantly useful, and I'm not happy either.