Thinking about the general question, I believe there is one environment where a signal mirror might be less effective - at sea or a similar body of water, at least for other vessels at your level. This is because it is fairly common at sea to experience "mirror flashes" from other vessels. With this in mind, aim several flashes in succession at an intended target - that would be unusual and should get attention.
Even then, a signal mirror is unlikely to work. While on a beach a month ago, I was imagining someone stranded out there on a kayak. When I paid attention, I realized that there were thousands of strong reflections coming off the water. The reflections while on a boat were even worse. I couldn't imagine anybody detecting a mirror flash as being distinguishable from all the other reflections. If a mirror flash were noticed, it would be mostly dumb luck I think.
I see your point about wave flashes - they are quite evident in the
WWII SIgnal Mirror Training Film (11 minutes runtime), for those of us who haven't looked lately.
However, the U.S. Coast Guard, whose job it is to rescue you at sea, advises that you carry a signal mirror with you when at sea, and there's lots of other evidence that it is a good idea (see below). So, I'm thinking there are mitigating circumstances here.
Perhaps some of the folks here who've worked maritime SAR can comment?
The U.S. Coast Guard Auxiliary says:
"the first item we recommend you attaching to your PFD is a signal mirror" U.S. Coast Guard personnel
aren't allowed to get underway unless they are carrying their signal mirrors (and EPIRB, flares, whistle ..., but the mirror is a hard requirement).
Commercial and passenger ships worldwide are required to stock their lifeboats with signal mirrors under the
SOLAS treaty.
The US Coast Guard daylight search guidelines call out a 5 nm sweep width when searching from a Coast Guard cutter when the survivor has a "sun signal mirror" vs. 4.6 nm for handheld orange smoke when the watchman is fresh, and 2.8 nm after he has been on duty for 3 or more hours[1].
(Per page 3-19 of the above, this seems to mean that they will try to lay out a search pattern that gets within half that distance (2.5 nautical miles for the mirror), but I'd welcome clarification from one of the former/current professionals here.
(Literally) boatloads of people have been saved at sea due to their signal mirrors being seen by other boats - in fact the U.S. Coast Guard history credits their requirement for mirrors to a 1942 incident where 18 were saved by a passing vessel thanks to a
mirror fashioned from a flattened tin canThe most recent example I have are the 23 rescued 3/9/2009 off Thailand when a small fishing boat
"responded to the flash of a special mirror". The "special mirror" was part of the emergency equipment on the life rafts (as per SOLAS).
The oldest example I have are the
50 saved from the Galway Castle in 1918. [1]
COMDTINST M16130.2E 21 SEPTEMBER 2009 (8.7 Mb)
U. S. COAST GUARD ADDENDUM TO THE UNITED STATES NATIONAL SEARCH AND RESCUE SUPPLEMENT (NSS)
To The International Aeronautical and Maritime Search and Rescue Manual (IAMSAR) Sept 2009
Table H-20 Visual Sweep Width Estimate for Daylight Detection Aids (Page Page H-38)
{ Calls out a 5 nm visual sweep width for sun signal mirrors, 4.6 nm for handheld orange smoke when searcher is fresh, 2.8 nm after searcher has been on watch 3 hours. }
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