It was discussed previously, but please find it here again:The point is that they actually understood that DM was not good sonic wise and they rectified it on the next album.
He praised the record around its 30th anniversary.
Metallica and Maiden are on two different paths, since decades now, in terms of satisfying their listeners. Metallica has to appeal to a wide audience, they decided to invest into a polished sound and based on how that band functions, they indeed need a producer in full mode.
Maiden did not choose that path. That's it. It's more than a sound issue, it's how they function.