I give REO an A for effort for his investigation on this...
And I know the Bone Shaper activists are saaaaaalivating at the prospect as using this as some kind of ammunition for luxury nerfs.
But, let's get a few things straight.
B3 does have 4 active frames, and he does have less recovery to deal with if blocked on the later possible active frames. That is entirely true.
However, in this case scenario, this is almost entirely Mileena's problem.
Mileena's roll, while active on the 8th frame, has poor travelling frames. The first few active frames will only really make contact when she's point blank as point blank can be, and it's why it has trouble punishing things on block in the -8 to -14 range. It's something I and others have noticed with one of my main training partners
@theGabStandard when he has played some matchups.
Something like Reptile's dash however, despite being active 1 frame slower, can punish B3 from any distance/active frames that it is blocked.
So yeah, it is something I've been trying to keep in mind for a few matchups, but everyone might wanna refrain from pulling the strings on the party poppers just yet.