Also, when you say "design problems" it sounds like you're asking about the technical challenges or technical flaws. A technical challenge would be when the designers have to figure out how to design a particular circuit. A technical flaw would be an error in a multiplier that went unnoticed before production. On the other hand you may be asking about architectural problems, which refers to problems with some aspect of the 8086 architecture, e.g. the segmented memory architecture used to be a big limitation that the Motorola 68000 did not have.