I really like Kirroughtree, definitely moreso than Dalbeattie or Mabie. Kirroughtree maybe benefits though from usually being part of a weekend break for me whereas the other two are more likely to get ridden as a day trip from Edinburgh.
Mabie often feels like a synthesis of other trail centres. It's nice but there's only one bit (a lovely fast flowy bit from the stream crossing to the start of The Scorpion) that I feel approaches "best in class". Dalbeattie has never clicked with me. Trekster showed me (and Northwind and another mate) some lovely natural stuff there recently but the waymarked/built stuff only really entertained me for a couple of sections. It felt like it was trying to have the same flowy feel as Kirroughtree but it wasn't really managing it.
Usually now if I make a day trip over to Dumfries & Galloway it's for Drumlanrig rather than Mabie or Dalbeattie. For Kirroughtree I'm willing to organise a trip, book accomodation and make a weekend of it.
I can see that Kirroughtree would be less fun on wobbly legs. That may have been some of my problem with Dalbeattie that last time as I was low on energy all day and there were a few pedally bits that may have been much better if I'd had the grunt to do something other than spin up the short climbs in it in the granny ring. However at Kirroughtree even if I'm not full of beans there's several bits where I'll manage to dredge up some energy to pedal enough to bring on the grins.
I don't find the amount of fire road at Kirroughtree excessive. There's some at the start of the red but the only bit that sticks in my mind is the long section taking you from the furthermost point on the red to the start of the black. There used to be a similar section on the return journey but that (and I think a few other sections of fire road) have been replaced with more singletrack since it's been opened. Personally I find the fire road at Dalbeattie more intrusive, perhaps because it seems to be more weighted towards the end of the ride - plus Kirroughtree has a much better finish than Dalbeattie, which helps make sure that rides there are remembered warmly.
Rocky Road at Kirroughtree is generally referred to as "We Hate Hardtails" by myself and the riders I'm usually there with.