Trung, as usual, is spot-on ... this is probably 2nd only to "Top ASA Bat" in that regard.
However, because I'm avoiding work, I'll sum up for you:
-There are manufacturing differences between FP and SP bats, but generally not as severe as you might think.
-One main difference is the handle is often reinforced because of different stresses on the bat.
-Another is that MOI/etc. are changed because of the different swings with and requirements for FP players - for some players, these differences may make a positive difference (often players with slower swing speeds, which will also be aided by a lighter bat). For others, FP bats may not work as well. [This info per tumblebug]
-Many FP bats are made in -8, so 34/26 (the exact same as SP bats) exist. They are legal everywhere barring league rules - "OFFICIAL SOFTBALL" and all that. Some, like VFPs, were even made in 27 and 28 (rare).
-While some may have durability issues, it's not the rule - OG Easton FP bats still act like OG Eastons, for example, and Catalysts last longer than you do.