Interesting question and topic. Actually, I don't think either is worth carrying for the usual hiker's circumstances.

At sea, a red flare is a universal signal of distress and vessels which sight it are under obligation to render aid or assist in some manner. on land, not so much (Honey,look, some fool is celebrating the 4th kind of early. Shore is purty!)

For a land bound hiker, weight and volume are more critical, and there are more effective alternatives which are also not likely to set the woods on fire (that capability might be useful as an ultimate fire starter, though). Modern electronics, especially the cellphone if service is available, are the way to go. Currently, a lot of emergencies seem to involve initial cell phone notification. But always pack a signal mirror!!

If you are out in the boondocks and hurting, you have to treat injuries or deal with the immediate threat, devise some kind of shelter, probably along with a fire for warmth and cooking/water purification, and get word out/signalling. Actually, your properly tended fire makes a pretty good signal and is likely to get the attention of the local kindly ranger.