The problem with buying a pre-fretted neck, outside of the slots not staying as tightly compressed on the tangs until it's glued are many.
1. You are now stuck with the size of the fretboard, this is fine IF your neck is the same. Yes, you can sand it down, but it's awfully hard to make it look nice when you have to take fret ends down with the wood.
2. I don't care how well your frets are pressed and how level they are, they will need levelling after you glue the board, because when the board backbows the slots open up a bit, if your frets aren't radiused to the board PERFECTLY, this can cause either ends, or centers to raise.
3. Quite often when you glue on a piece of wood such as a fretboard, the glue will cause the wood to swell, then shrink back, usually it will shrink back a little further than when the fret ends were dressed, so now you have to redress them anyway.
4. If the frets are not radius'd exactly it can literally cup the fretboard, there is a lot of force in a radius'd fret. You get 24 in there, and yup, the board can cup. then it's harder to glue it onto the neck.
There are companies who fret first, Seagull is one, they also have a vacuum press to glue their boards on, and every single neck and board is CNC cut so the width's etc are identical, they also have fret edges that stick over on their guitars in the store.
The hardest part to get correct in fretting is dressing the ends and levelling the frets. If you fret before you glue up, you're going to have to do this step anyway, so I wouldn't let pressing the frets deter you.
The ONLY time I might consider fretting first, is on a board like Gibson or some Fenders where the fret ends have to be cut to 90 degrees because the binding follows the edge of the board up around the frets. But then, I think that binding system is stupid anyway.