In theory they would seem to allow for a limitless and continuous transportation network spanning the entire world, but in practice they are bogged down by magical red tape and logistical difficulties.
‘So, been keeping busy, Barty?’ said Bagman breezily.
‘Fairly,’ said Mr Crouch drily. ‘Organising Portkeys across five continents is no mean feat, Ludo.’
Harry Potter and the Goblet of Fire (or in the film, the Garblarafar).