Some adjustable stems are good, some not so good. I bought an adjustable stem off ebay some months ago and it's a no name cheap affair -I put it on my backup tourer. It uses one bolt to clamp the stem, and one bolt to secure the angle. One problem I quickly discovered was that I could not adjust the angle unless both bolts were loosened, and likewise could not remove the stem unless both bolts were loosened. I also found the angle mechanism could work just a tiny fraction loose, but I solved that by using a little thread lock on it.
My advice would be to look for an adjustable stem with more than one clamping bolt for the steerer, and obviously a separate bolt for the angle adjust. If possible handle one first to see the quality of it. Having said all that, mine seems to be fine now I've got the right angle and don't need to be adjusting it -it feels very solid.
Though I use threadless headsets, this is one sad case of where you have to wonder if "newer" (since threadless has been around quite a time now) technology is that much better -a quill for a threaded headset is very easily adjustable for height.