I full-length re-size my .300win mag brass all the time. I have ZERO problems doing so. I use a basic LEE PRESS. I do keep a watchful eye on the area about a full centimeter above the belt. I take a paper clip, straighten it out, then scrape it on the inside of the brass near the belted area. If it begins to thin in that area, a small ridge will be able to be felt with the paperclip. (the edge of the end of the paperclip is quite sharp, and easily feels any discrepancy in the brass, and you will feel it catching on the ridge)
I'd be more worried about the high pressures associated with RUM rounds, and think that brass would undergo ++ stress. Neck sizing would be very beneficial to RUM brass I think.