Everything has it's faults.
Cannondale - back in the day when you saw a roof rack on a newish BMW it had one of these on it. Very rarely with any mud... just for posing! I also never got the handling - dreadful.
Klein were even worse, they were just never taken out of the garage which is why so many still look like NOS. Bought by complete posers for talking about and not riding.
GT - always hated that tripple triangle groove tube blah blah blah marketing pish. Always heavy.
Marin - the Zolatones where cool but after that it all went superlight and fragile. Also tended to be bought by a particualar kind of person who had a beard and did lots of research and found that the Marin was the lightest and best and and and... yawn... but they didn't realise quite how Marin had done it... the wheels and other components were too fragile to be ridden and the poor shop had to fix everything.
Trek - too much marketing and not enough thought/engineering in the product - Ie bonded frames, and how many really crap suspension designs have Trek had?
Kona - for all the hardcore image why could they never take a tyre larger than 2.1, and that rear brake thing round the seatpost was always crap.
Specialized - Stumpjumpers and Rockhoppers were nice but the Hardrock and lower end stuff devalued the brand.