The first Xbox was really reliable (at least more so than the PS2), so whilst it may not be contributing to the 360's success I don't think you can generalise and state that they always make faulty consoles.
Indeed it may be the popular perception, but people forget quickly - the PS2 is worthy of many accolades but 'reliable' isn't a word I'd choose to describe it. That's why I have a hard time 'pigeon-holing' all their woes to an issue of failure rates.