...I can't shake the feeling that the amount of perks and peril sunk into this particular build aren't really worth the payoff. On the other hand, it sure is fun, so I think I'll keep experimenting with it.
This may come unexpected, but MFa is actually more of a defensive build than an offensive one (that's MAD). Whereas MAD uses what is called tactical offense, which plays the game of hit-without-being-seen, MFa uses active defense, which plays the game of move-without-being-hit. It's a little hard to understand until you reach the point where death comes very quickly to the unprepared. On the easier difficulties, MAD is typically a better shotty build...if you're using ONLY shotties, that is. MFa is a great advanced shotty build because of its active defense capabilities, but also because there's a lot more you can build off of it, such as WK, Iro, or even Badass if you want heavy defense.
(Regarding Thomas's comment, even if the knockback invincibility weren't true, most of the time you'd still dodge the R/M/V attack and only get hit by the splash anyway, thanks to HRx2/DM. The key, then, would be not to run directly toward your opponent and get as much out of sidestepping as possible.)
Also, speaking of traits and stabbing, it seems Whizkid doesn't work on combat knives? Is that a bug?
Melee weapons get the same maximum mod capacity as armor and boots. That is, for 0/1/2 levels of WK you can add 1/2/3 mods (as opposed to 1/3/5 for ranged weapons).