Multiple modes are an illusion. No one designs a character to use a longsword two-handed on purpose. It is an emergency tactic that never happens. Flex makes the benefit happen 100% of the time, which is great.
Monks are a class that exist. A warrior class, even. And flex is the mastery attached to their most commonly used weapons, especially at low levels, quarterstaff and spear. Shouldn't it be designed so that they can benefit from it? But, as usual, let's proceed as if fighter is the only warrior class that exists.
Without Flex, a Champion Fighter is dealing:
1d8+Str at level 1, up to 4d8+4xStr at L20.
With Action Surge, it is 2d8+2xStr at level 2, up to 8d8+8xStr at level 20
With Flex, a Champion Fighter is dealing:
1d10+Str at level 1, up to 4d10+4xStr at L20.
With Action Surge, it is 2d10+2xStr at level 2, up to 8d10+8xStr at level 20
With both, the Champion gets critical hits on a 18-20, doubling the dice. Doubling d10s are better than doubling d8s.
That is GREAT when all those attacks are used against a single enemy. Especially with there being so many ways to get advantage in the game.
This is fine compared to the Battle Axe's ability to Topple, which can knock them prone once. If that Champion Axe Fighter has Weapon Adept at level 13 (able to have 2 masteries at once for his battle axe), and Topple is useless for every successive attack because the enemy is already prone and they have advantage for the rest of those attacks, is a Battle Axe Champion going to choose Graze over Flex in this situation? I would choose Flex every time.
Flex is FINE.
Well, let's math it out. We'll use a level 11 champion, and we'll create an opponent who has AC 18, which seems like a reasonable number for level 11, and +6 to their constitution save for when we test topple. This champion will have maxed strength, so they are at +9 to hit.
With flex, the math is pretty simple: 18.81 DPR without flex, 20.79 DPR with flex. Flex adds 2DPR. That's including critical damage on a 19-20 (the value of critical hit damage is VASTLY overestimated by most players; it's fun when it happens but when you math it out, critical hits don't actually affect DPR that much).
With topple, it becomes more complicated depending on when you hit and whether the creature makes its saving throw. Then, the benefit of topple depends on how many attacks are made on the target while prone. So we have to run some different scenarios.
Scenario 1: 1v1, fighter topples foe on first attack: 23.83 DPR - topple is slightly better than flex.
Scenario 2: 1v1, fighter topples foe on second attack: 21.53 DPR - about the same as flex.
Scenario 3: fighter topples foe on third attack, or fails to topple foe: 18.81 DPR - slightly worse than flex.
So in a 1v1 fight, flex is about the same as topple in terms of raw DPR, though the actual benefit of either is pretty small...when it comes to damage. However, flex gives zero tactical benefits, whereas having your opponent prone is obviously very tactically useful, and so I think any sensible player would take topple.
However.
D&D seldom comes down to 1v1 fights vs. the BBEG. Instead, we have to consider the advantage that the mastery gives to the entire party, and here it is no contest - the benefit of topple is magnified by the number of attacks made by the
entire party, on top of the tactical advantage of having the opponent prone, whereas flex is just a small increase to your DPR (when you average everything out, especially the chance to hit, you'll find that flex adds about .7 DPR to each attack, and you can multiply that times as many attacks as the fighter, and only the fighter, makes - so in your best case, level 20 scenario above we are looking at about + 5.6 DPR on the action surge rounds, about +2.8 DPR normally). Even in a world in which you never miss and crit on an 18-20, flex only adds 1.15 to each attack. That just doesn't compare to what other masteries offer.
So when WotC talk about flex being mathematically strong, I think that is just not supported by the numbers. And, again, that's setting aside the fact that the warrior class who are mostly likely to be stuck with flex, monks, can't use it at all, so their mathematical advantage is zero.
Edit: Conclusion: Flex is bad. It offers a slight increase to DPR at best, no increase to DPR to the class most likely to be stuck with it, and zero tactical benefits. The player base is correct to give it a low rating.
Edit 2: fixed some math - topple is actually at least as good as flex 1v1, just in terms of DPR. Flex is even worse than I thought.
