It's a bit faster than an FdL, depending on your loadout.
My Mamba is a whopping 10m/s faster than my FdL, both built for PvE combat.
I doubt that's enough of an advantage to offset the extra 200 integrity and extra 200MJ of shield (from the same shield generator, btw) that the FdL has and convince people it's an equally good choice.
This. You take the exact same modules from my Prismatic FDL and put them on a Mamba, it goes from 560 to 582, with about the same shields, but a trade off in the optional internals because they aren’t of an appropriate size to support SCBs that will recharge the shield in any meaningful way (unlike the FDL that has a size 5 and two size 4s, allowing a a lot of options in this regard).
It has no true niche right now or a reason to exist other than aesthetics. Build it to reflect its description though, and we have a winner.