These forums are full of people who have found small wheels satisfactory for more diverse riding. I hear of people saying that Bromptoms are slow and poor handling compaired to a road bike. I got a mezzo to preform close to a road race bike and a Brompton to be quick and handle very very well indeed. I have a Birdie set up which works superbly off road. More comfortable than my quality full sized MTB.

I don't tour, but many other find the correct 20 inch folders very practical and enjoyable.
Many people also tour on Birdies and brommies.

It depends on your need for performance, values, and importantly wether you enjoy the bike you are riding.