I don't want to carry two radios that are identical. Do you know which radio works for everything?
As you've been told, no radio will
legally work for everything. You can use any radio on ham radio if it is within certain specs.
FRS-only, GMRS-only, FRS/GMRS combo, and MURS each have certification requirements that disallow mixing bands on the same radio, and none of them allow ham radio capability on the same radio.
It may be true that some radios, such as a Baofeng UV-82, UV-82, UV-82C, MURS-1 and GMRS-1 are all
identical radios except for firmware settings that restrict the channels and frequencies and power settings, but you're still not legally allowed to use them on bands they are not certified for. The FCC rules are kind of stupid, behind the times, and a lot of people just flout the rules anyway. I'm not advising that you do it but the likelihood of getting caught is infinitestimal unless you are operating in a fixed location for a long time
and causing harmful interference that gets reported and investigated.
I will note that the use of Part 90 radios on GMRS is
widespread. People talk on and on about their Part 90 radios freely on all the networked GMRS repeater systems. Most of the repeaters use Part 90 transmitters. No known FCC enforcement. The FCC apparently just doesn't care. And why should they. Technically, Part 90 radios that cover 462/467mhz are perfectly good radios for use on GMRS. Again,
de facto it's all about harmful interference. You hiking with a Part 90 radio and using it on GMRS and ham and MURS just isn't going to matter. Legal disclaimer: This is my opinion and I am not advising that you do it.