I like trail centres. You know what you're getting. They have also been specially built with drainage in mind. This adds to the reliability factor.
Natural stuff is great if you know the area, and as such for me the peaks is absolutely great once in a while when it's not mudded up.
With natural riding you have two main risks,
(1) is that you can use your OS map to plan what looks to be a great route round the countryside but when you go round it, it could quite easily turn out that you're hacking along a boring farm track for 90% of it, or having to do lots of road descending
and
(2) is that most, or part, of it will be absolutely mudded up. Some people like a bit of mud, I don't mind a
little bit, but ploughing through a quagmire:
no thanks. This, thankfully, hasn't been quite so much of a problem recently though as it was in the years of freak rain of 2007 and 2008.
It's just the reliability aspect. For me I can drive over to the peaks but it takes best part of an hour and a half, but I can be at dalby in not much more than that, a lap of which is over 2 hours of solid riding with a much better ratio of riding to road work, and what fire road there is is up hill so you don't even mind that.
YMMV, but for me, MTBing is fun for largely different reasons to what road riding is and it isn't just about "getting out there and seeing the countryside".