I have never read Mythus and I doubt I'll read it in the next month or next year.
Gygax was not the author who influenced me the most among the great geniuses of the past of the hobby, although I would not dream of belittling the extent of its importance.
But I digress... and this is not the place to rehabilitate the other creator of D & D.
I've never played Mythus, although I have the manuals. I hear that Gygax wrote them in a style quite convoluted and complex.
I remember seeing in the past ads which suggested Mythus was a game better than AD&D because Gygax had surpassed himself.
If so, the matter would be very interesting, because it would mean that in the eyes of Gygax AD&D was not perfect, nor was the maximum achievable in the field of fantasy role-playing.
But I admit I do not know the reason for its publication, perhaps they were related to the fact that Gygax did not work at that time at the TSR?