An 18 inch wheel does not really work very well off road, but fitting a bigger (and taller) tire helps.
It adds weight though, making it hard to loft the front wheel over things, at least on the TW200 with its limited power.
On the TW200 with a good big front tire (the stock tire is very wimpy) you can hit almost anything and just not feel it much.
On other bikes, I always (tried to) avoided roots and so on, with the TW I just ignored them.
I have tried dual sporting bikes with 18 inch front wheels, and they gave a very rough ride.