I"ve read (and planning to play) Against the Darkmaster; it is certainly inspired by MERP (it states so outright), but it is just as certainly not a knockoff. It uses the core principles of MERP, but everything is re-designed from the ground up, and new systems for passions/drive, travelling and lots of other new elements are introduced. The magic system takes its cues from MERP, but is very much its own thing. All in all, AtDM is a slightly more narrative take on the RM core, but still very crunchy.
AtDM is written in the OSR spirit of taking a beloved, classic rules system and doing something new with it; it is very respectful of its predecessors, without copying anything, and it shows that it is a true labor of love. And from where I stand, I can't see it hurting ICE, I would suspect that ICE will actually benefit from it. The game has once again rekindled my interest in the RM family, and a lot of people looking for adventure/setting material for AtDM will probably end up buying HARP and Shadow World products, which are relatively easily compatible. Just look at chaosium's Basic Roleplaying; by now, there is a huge ecosystem of games based on it, by dozens of publishers, and it looks like they mostly all benefit from each other. If I wan't to play on the official RuneQuest world Glorantha, but the rules are a little to heavy for me, I can get the RuneQuest Glorantha Core Book, but buy OpenQuest from another publisher as well and use that one.
Personally, I want to use AtDM with Shadow World, so me being won over by AtDM means me buying Shadow World products.
EDIT: I did a series of reviews on AtDM, starting here:
http://swanosaurus.blogspot.com/2021/03/reading-against-darkmaster-part-1.html