I'm going to play devil's advocate here and say that practical experience and personal testing indicate otherwise from my perspective. Do it right and it's secure. Not ideal, no, and I wouldn't recommend it to anyone with a feasible alternative option. However, if you need to do it because you stripped the fixed side of your hub (what I did) or something like that, you can do a good job of it and have it be secure.

Last time I did this, I ran my rear wheel fixed in this manner for a full year. When it came time to take the cog off, I decided to see if I could deliberately back it off using brute force. Broke my chainwhip testing it that way, and then my actual chain attempting to rotafix it off. Then, I fixed my chainwhip, took off the lockring, and removed the cog without issue.

The lockring doesn't just thread up against the cog in this setup, it exerts a great deal of force upon it as well.

Again, though, I don't recommend it to anyone who has another option, only as something that can be done if need be.