about what?
mind you i really like my school, just didn't like the message they sent.
i do think the methodology is a bit skewed towards schools in richer countries. a Portuguese graduate who gets a job at, say, the local office of a top international consultancy, probably makes less money than an French graduate at a local consumer goods company. it's not just about salary, it's also about cost of life where you work - Lisbon is probably the cheapest capital in western Europe. even the 'best' graduate jobs in Portugal will probably only pay as much as an average placement in other WE countries. this isn't reflected in the ranking.
'quality of employer' (reputation, wages, selectiveness) is another one, but it's damn near impossible to quantify so doesn't figure into rankings. plus, maybe you just want to work at a less selective company.
one could argue that the best graduates from any top university would get jobs in central business cities - London, Paris, etc. - so they'd get a high 'central' wage, regardless of where they studied, but there are countless reasons why one would choose stay closer to home.
but between the usefulness of university rankings and the methodologies behind them, we'd be here all day 😛