I really think people discount riding long distance on Bromptons for the wrong reasons. For touring, they are incredibly practical. I did my longest rides so far with the original saddle, although, I have worn one out, and still deciding on ideal replacement... the decathlon saddle I have on there at the moment is OK, but not put distance into it yet (but it was bad on the road bike on the turbo for multi-hour sessions), but seems OK with the titanium seatpost on the brommie. But out of the box, it's absolutely fine for long distance, if the saddle is comfortable enough for you. I might prefer something firmer now, but that one did me fine for many thousand kilometers, and does for many people, otherwise, I'd suggest Brompton might've changed it. Being able to stick a cateye mini in it is a nice bonus.
What I have learned in the last few years, is that getting tyre pressures in the sweet spot does help. While you can go pretty high, it does affect ride quality and rolling resistance, especially on certain rough road surfaces with the smaller wheels, so too high, you even have to pedal downhill on some surfaces, just to maintain speed. Unless all the roads around you are super smooth, 80psi as max is fine. But below a certain level, especially with Marathons, it quickly turns into riding through treacle. I really like the conti-urban-tyres at the moment, that seem to work well in all weather, and need to get quite low before they start to feel slow.
But then there's the luggage. Things like mini-O-bag don't have massive wind resistance and great for the during the day stuff, and if you're a small guy like me, then sticking a 15L+ seatpost bag on a Brompton is never going rub on the rear wheel like is a risk on road bike. But then, also being able to add another 20L plus with a bigger bag on the front luggage block gives you a load of storage. That's before even adding the, arguably, useless rear rack, although, might be useful for some stuff, I haven't really felt I've missed out by not having the weight of it. Sticking the same amount of stuff on a road bike for me means 3 separate bags, at least.
Then there's the having to stop on a steep climb, which I've had to do more than once. Easier to get a Brommie going again than a big wheeled bike. Perhaps easier to keep bigger wheeled bike front-wheel on the road if climbing seated, but WAY more satisfaction from doing that sort of climb on the Brommie in the first place.
A well maintained Brompton I would have no fear about riding over long distance. Quality bike, with other qualities.
And I will also add, that my cheap road bike didn't really feel much better until I switched to some nice Hutchinson Fusion5 All Weathers run tubeless. Then, it really did become slightly different level of comfort on the road (WAY quieter too... no distinctive rattle of a Brompton).
