The safety issue depends on the gun's tolerances being right. I would not personally load an M14 with neck-sized-only ammo. Neck-sized cases grow a little with each firing, getting tighter and tighter, so the risk of a slam-fire increases with each loading and firing cycle.
At large matches (several hundred competitors) I've been present for a number of slam-fires over the years. There is also member on this board whose username is Slamfire, who has had two out-of-battery slam-fires that destroyed the guns he was using and that caused him a degree of injury. These were in M1 Garands, but the Garand action is the parent mechanism to the M14/M1A mechanism and the slam-fire sensitivity is similar. You can look up Slamfire's posts on the subject.
I'm sure there are people who get away with neck sizing in these rifles because random probability says there have to be some. Probably, it works out most of the time. Just not all the time. So you need to be aware you are taking an added safety risk as compared to someone who sizes to set the case shoulders back. I don't personally consider it good practice to incur a risk that can be so easily avoided.
More than the above, benchrest shooters using body dies to "bump" shoulders back about 0.001" have found it allows some self-alignment of the case that actually shoots more accurately than neck sizing alone. So you may have some benefit to light resizing that also improves your safety margin.
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