Cross company travel may not be much of an issue in the longer run, as the transit apps are not restricted to be offered by the companies themselves, so the apps offering an integrated view should win out.

Hoever, planning e.g. a trip from London to Paris, with local transit on both sides, may be difficult.

Not Paris, not Rome.

Found the list of cities. It's massive. It's true no Paris or Rome, but many other cities in France and Italy. http://www.google.com/intl/en/landing/transit/text.html#eu