They are generally okay to machine, I used tips I got for stainless and they were fine. The only thing I found is that the diameter will vary a little if you are only taking a few mm off as the surface hardening is about 2mm deep. This means when you get to the diameter where the thread is just gone, you are still left with hard spots 2mm deep where the root of the thread was hardened. I'm not explaning this very well but think of two spirals of hard and soft material intertwined. If you can go 2mm below the root of the thread with your final OD then this effect is gone. If not it leads to the tip being pushed back more by the harder bands than the softer ones so don't be surprised if the same diameter checked across a couple of different point will vary by up to 10um or so.
I take shafts down to the final dimensions with 600 grade paper and this reduces the variance nicely.
Shafts will vary in terms of hardness and depth of hardness but there is nothing to fear either by taking on the task youself.
Rgds,
Noel.