Having used a variety of tools to deflate tires, including Stauns which I still have and still use, I still believe the fastest inexpensive way to do it is with a core tool, one tire at a time. Practice makes it easy to guess at the current pressure, a gauge confirms the current pressure without the need to reinstall the core, and if you start putting the core in a few pounds above where you want to be, you only have a little bit to adjust and you are done.
The Stauns are relatively easy to use, but don't hold constant pressure settings over time. Dust and grit interfere with the internals and will give you erroneous results, including a completely flat tire if you don't catch the problem. The Staun units need to be maintained from time to time, and kept in a clean storage place. I have used only one Staun to deflate all four tires and come out with different pressures on all four tires. So, I use them and like them because at times they are convenient, but don't really rely on them to set pressure. I use them to get a pressure one to four pounds above my target, and do the rest with a pocket tire gauge. Having used them over and over for years now, I think taking the cores out is almost as fast as using the Stauns if the amount of pressure drop is significant or if you have large tires.
I can't think of a reason why you can't drive -- on the trail -- with the Staun unit releasing pressure as long as they are clean and working well. Done it lots of times, but you have to trust the unit to work as expected, and you should stop and remove them just as soon as enough time has gone by to deflate the tires, which means I would only do this if I was in a hurry to get moving. I would not simply leave them in place during a trail ride. They can burp air and if submerged, can allow water to enter the tire. They are also on the long side and can get stripped off by trail debris. Better to be patient, deflate and set pressures, remove the units, then drive on.
