I was watching some videos for the video thread recently and thought about this while watching Nezu play (or who I think is Nezu).
The main point of it still stands as pretty much a free guessing game because I'm sure most people can't react fast enough to Bang's FRKZ mix-ups in that particular situation even after an IB (unless they are mashing). So in that case, they still have to guess right out of "quite a few" choices the Bang player can make. Even if they guess right once, in most cases I'm sure the Bang can almost go into another mix-up immediately after so it is another guess they have to make (but I guess that also depends on what they go for after the IB). During the FRKZ "start-up", I would say the Bang player is definitely in control of the mind games at that point, whether it is during a block string or not.
In one instance, I saw "Nezu" just do 5A into tick command throw, they IB'd the 5A and he still caught em with the command throw probably from them not doing anything even after the IB'd 5A from being scared of Bang's FRKZ pressure/mix-up.
While it may help out the opponent by giving them IB's, I don't think it helps them enough to completely disregard doing FRKZ even during a blockstring. I could literally do the 9dash-j.C after FRKZ startup in like 5 straight rounds and I'm sure most people still wouldn't block it. People either can't react fast enough to it, or are too busy thinking too hard or being scared and get caught by it 9/10 times.
In my personal opinion, I'd say don't let this mechanic stop you from doing it, but definitely remember it is there. Besides that, I think there are plenty of opportunities to use FRKZ outside of a blockstring (and still be to your advantage) to where you might not even have to worry about it.