The Fafnir series has been one of my favorite units since the day I started playing Battletech, and I've tried fielding one whenever my opponents would let me.
The 6U, my personal favorite, actually lags behind other variants in terms of overall effectiveness. The unit moves too slow to garner a good TMM; The Light Fusion Engine combined with explosive ammo results in undue fragility; And the Light PPCs are not effective enough as backup weaponry on the machine, although they may deter lighter units. While the unit excels at close range city or forest fighting, even with the lack of fists, it is very much a victim of environment, and suffers in anywhere with more open firing lines.
That being said, the 5WB is far more practical in comparison. That machine is just terrifying.