In terms of being let down by what should have been a legit frame and company, the ONLY bike (and company) I have had a bad experience with was a 2010 Marin Mt Vision.
I bought it as a frame and built it up. Never liked the suspension characteristics. It supposedly had 125mm travel, but it utilized it very poorly. It had the sag and wallow of a 140mm frame and the bump absorption of an 80mm frame.
But the real kicker was that the bearings were shot practically out of the box. These were advertised as aircraft grade and guaranteed for life. But the attitude of the Marin service rep was basically that these bearings are so good that if they go bad it must be something else responsible, and they would not send replacements.
I have owned 6 FS frames:
2000 Santa Cruz Superlight
2003 Heckler
2005 Azonic Saber
2009 Iron Horse MKIII
2010 Marin Mt Vision
2012 Turner 5 Spot
I can say I liked all of them except the Marin.