Difficulty? Also how viable was the build before the masterwork fauchard?
Hard so far. I've been considering Unfair, but I kinda fear the tedium of keeping the squishier party members alive. I had to hide Linzi in the corners in the Pitax palace already.
Also DC spells land rarely enough as it is.
The build was extremely viable throughout. In fact I respecced from a Hungerseed Tiefling to a Motherless when I realized that Bite attacks stack. Had I played a Motherless from the start, it would have been even more easy (4 full BAB attacks with high Str & bonus sneak dice at level 2, 2 bite & trip attempts per round from beginning of Act 2 and Guldrin Bridge visit).
Actually at least till you get the Serpent Prince fauchard, the 4 max BAB natural attacks from Feral Mutagen and Motherless are a much stronger attack option.
Even then its debatable, at least till you get Improved Critical Fauchard. Before that another option is to flurry a quarterstaff with high Str bonus (but you incur a -5 AB & 1/2 Str damage penalty on Bites using manufactured weapons).
Also a respectable AC with the Monk level, good Wisdom, Mutagen Natural Armor boosts, decent Mutagen buffed Dex, Shield spell, monk robes, +1 Int AC from 1 SS level, later also Dragon Disciple bonus Natural Armor (now about 60 buffed - without Defensive/CE or flight bonus).
You also have a high ability to self-buff, including Improved Invisibility, Shield, Barkskin, Haste, Enlarge, Blur, Displacement, Death Ward, False Life, so are pretty self sufficient (and can trigger sneak attacks even without flankers). Late game I've even learned Mirror Image (from the Bard spell list).