Do you know of any reliability issues with the FF?
Oh boy.
The FF has its share of reliability problems. Maybe not more than other portables, but certainly not fewer either. I've been through quite a few units myself, for a variety of failures. If you read the FF thread, or even the "best of" thread you'll see a lot of these issues. (On the plus side they have fantastic customer service, so the reliability issue isn't stressful, IMO.)
Have you ever tried concentrates in it?
I have not, but I have the pads. I've heard mixed things about them. (Again, do a search on the FF thread. I think this might be covered in the last dozen pages or so.)
And if you had to own only one vape, which would you choose between the two of them and why?
This is an extremely difficult question to answer because for me these two vapes are highly complementary. The FF is beautiful, sleek, on-demand, all convection, one hit or session, very weed-conservative, very fickle learning curve, often fussy even for veteran users, and of course, amazing flavor. The Mighty is machine-like, brutal, more session-y than one hit, a bit of conduction mixed with convection (not enough to notice though), uses more weed, zero learning curve, works perfectly and thoroughly and unbelievably simply every time.
If forced to choose, truthfully the Mighty gets a lot more use at my house. It's just so damned easy and amazing and consistent. But the FF gets a lot of play too, and when you just want a couple hits at peak flavor, it's the master of that domain.