Here is my two cents:
The big 3 are difficult partially because the materials isn't as tangible (SPD is, but not the others) and the questions I got were scenarios where you had to decide the best course of action according to NCARB's opinion. And typically when people take these three, they are taking them in the beginning of their testing sequence (aka: when they aren't as familiar with how NCARB words things or how to think through the problems the way NCARB wants you to.) BDCS, SS, and BS were easier to me in that they were 1) about things that I had some familiarity with already from my work experience, 2) testing on tangible scientific proven information (typically not some opinion NCARB has) and 3) overlapping, but still distinguishable from each other which made it easier to study. Also, by now, you've gone through the ringer with the first 3 exams, so you know what to expect. The big 3 had a lot of overlap so it made it hard for me to figure out what I should even be reading! Yes, there is a lot of detailed information to learn for BDCS, SS, and BS, but I feel like it has certainly made me more confident at my job and when I talk to my MEP and structural consultants. And isn't that what we are really here for??
But obviously you have to do what is right for you and what you're comfortable with.
Also, I kind of wish I had done what gbalaka had done and saved SD for last!