I'm interested in looking at average damage comparisons for when it does max damage for taking disadvantage. What is the probability of hitting with disadvantage compared to the probability of critting without disadvantage?
I looked at this some yesterday. Let's say you do die-type + bonus in damage, abbreviated d and b respectively.
Then the average damage you do is d/2 + b + 1/2. (Technically, it'd be 19/20 times that plus 1/20 times (d + b), but the difference is negligible for the amount of effort it adds. Do note that including crits would inevitably make Battlerager just a tiny bit worse, though.)
If the chance of hitting an opponent is p, then the chance of hitting with disadvantage is p^2.
So with Battlerager, your average damage is (d + b)*p^2.
Whereas without the feat, you deal maximum damage with a probability of 1/20 + 19/(20*d). Or just 1/20 if you're only interested in crits.
So for Battlerager to make a difference for you, you'd need:
(d + b)*p > d/2 + b + 1/2
Unfortunately, there are three solutions, depending on whether p is >, <, or = to 0.5:
p=0.5: b < -1. In other words, if you have even odds to hit, Battlerager gives you a net benefit only if your bonus is negative. Since this is extremely unlikely for a melee combatant who takes this feat, Battlerager is a net loss under realistic circumstances.
p<0.5: d < [1 + 2(1-p)b]/[(2p-1)]. Assuming b is positive, d has to be a negative number, which is impossible. So for this case also, Battlerager is a net loss.
p>0.5: d > [1 + 2(1-p)b]/[(2p-1)]. Here at last we have a case where Battlerager has a net benefit. Let's take the worst-case scenario of p=0.55 and assume a bonus b of +3.
Then d has to be greater than 37, which is impossible in D&D.
Now take the best-case scenario, with p=0.95.
Then d has to be greater than 0.072, which is true of all D&D weapons.
Where's the breakeven point? If d is 12 (a greataxe, say), then p has to be 0.633, or in other words, you need to hit on a 7. This doesn't change significantly if your damage bonus is 4.
If d is 8, you have to hit on a 6.
In short, maximum-damage Battlerager is a losing proposition on average, except for easy-to-hit opponents. One could argue that this is realistic - that berserking is a losing proposition on the whole against armored opponents - but it doesn't make the feat look all that attractive.
However, Battlerager gets a lot more attractive if your bonus goes negative - in other words, if you've been debuffed. That's something to take into consideration. And of course, if you're at a disadvantage for any reason, the feat suddenly gets completely awesome.