Has there ever been confirmation from a developer that guild chests get the 4x/10x bonus to legendary/mythic troops that gem chests get? People keep saying it, but nobody has ever linked me to a dev quote when I call them on it.
40k seals chest is 0.108% per chest to get a Mythic and gems chest is 0.096% per chest, so seals chest is actually slightly better.
Taran’s website has the drop rates, you can check and compare glory chests & 20k chests there. http://www.taransworld.com/Chest/
In the initial preview video, Sirrian made a comment that 20K guild chests were “like” glory chests and 40K guild chests were “like” gem chests (with the exception of guild guardians, of course). So, that’s why people keep making that comparison.
As for the OP question about mythics…
This is anecdotal obviously, but I have gotten over half my mythics from 40K guild chests (without guardians in it). Scorpius just dropped for me after using 3,000 guild seals.