tdister is right. The Fenix P3D with red filter appears dimmer than the Dinotte taillight (I have tried both).

Some other negatives of using a Fenix light with a red filter vs. a Dinotte (other than lack of strobe mode already mentioned):

Relatively narrow beam. Viewed straight on it is very bright, but at a slight angle it appears dim. The Fenix must be aimed perfectly in order to achieve the desired effect. The Dinotte provides a much wider, brighter beam which provides more margin for error in mounting and better overall visibility.

The filter is fragile. While I was experimenting with mounting options for my P3D with a red filter attached, it fell off the top of my rear rack onto my concrete garage floor. The filter lens shattered. I have dropped my Fenix lights onto concrete from similar heights (and higher) and the clear lens does not seem to have this problem.

Of course the Fenix with the red filter would still be brighter than most of the taillights out there, and if you've got an extra one laying around and can figure out how to mount it properly the filter could be a good cheap investment. If you ever drop your lights, I'd recommend buying at least a couple of the filters since a routine drop could result in a destroyed filter.