That's a good question but I would bet you can still get achievements since resetting the nether is an in game option and it does not give you a warning message about disabling achievements.
For some reason Wither Skeletons do stop spawning after a period of time. I have no idea why. I also know that I have only found one skelton skull and have been at it for over 4 hours. Thats totally ridiculous.
Resetting nether