My work bought maybe 5 iBooks, and they ALL failed, mostly the CD drives, but also screens and hard disks.
If I had to guess why the cheap ones fail more than the pricey ones, it's probably the small size, overheating etc. My old MacBook (which had an array of issues) ran far too hot to use on my lap.
Macs are no more likely to fail than PCs (anecdotally at least, they fail less). However, internally Macs are just the same as PCs nowadays, so your only hope for greater reliability is better parts, or better build. The build of less expensive Apple notebooks at least has historically been very bad, the build of the expensive ones is historically very good.
The quality of parts, well Apple can be the victim of what they buy every bit as much as any other PC OEM.