I strongly suspect that the success of an upgrade, or multiple
upgrades, is what's been done with the OS in the meantime. If it's
just been used 'as is' and only basic programs installed from the
repositories then the chances are higher than an upgrade will go
cleanly. Once programs outside the basic ones or using external
sources are installed then upgrading is more likely to crash out.

On a similar note - there's been a lot of bad press about Win7 SP1
with some users having trouble installing it, or it breaking Win7 if
it does manage to install. Again - on a clean install of Win7 the SP1
update goes in without a hitch.