Just that they are not built the way "traditional" thinkpads are. I don't have personal experience how durable they are, but in general they aren't as easy to repair as T-series (for example). With T-series it's pretty straightforward to replace nearly every component separately (wifi, ram, keyboard, display...) which is either way more difficult or straight impossible to do with X1. Also spare part availability is generally better with T-series.

But to be honest, newer T14s aren't as service-friendly either as older T-series. But if repairability is a deciding factor then I'd skip X1 and get some T-series instead.