I don't think missing or misfitting parts is a common problem with Ikea -- I've certainly put together a fair amount and haven't run into those problems.

TBH, I think many people don't have the patience to follow the instructions. Even with the wordless ones Ikea uses, it can take a few minutes of study to understand the exact orientation of pieces a diagram is calling for.

I actually find wordless instructions much harder to follow; insofar as Ikea builds.are difficult (they rarely are particularly, mostly just time consuming drudgery which isn't the same thing), it's because of rather than in spite of wordless instructions.