amon hotep is overpowered because it combines invulnerability with high magic resistence and awe. The fact that it can also give you a free mummy each turn is just an added bonus.
amon hotep is nowhere near as useful as soulstone. all the others are nice, and useful, but nothing combines both very valuable ritual attacks and the most overwhelming battlefield magic. Its always amongst the very first artifacts to be forged... the others, like gatestone and amon hotep, can stay around being unbuilt for ages.
Arguably, that's because it's rarer to have the magic paths required for gatestone and amon hotep - you almost have to have astral on your pretender, and most people avoid that because of the Magic Duel issue. (But I know I often plan to get those in a long term game )
__________________
Wormwood and wine, and the bitter taste of ashes.