I don't think it was a great adventure for new DMs even when it was released.
Is it an adventure I adore and have run many times? Yes. But a good adventure for new DMs? Not really.
Keep on the Borderlands has a fundamental flaw: The Keep, which isn't described in a way that someone new to DMing has anything to work with. And then it puts a massive barrier between the players and the fun bit (exploring the caves), because they don't know where the Caves of Chaos are.
I'm tossing up between The Sunless Citadel and Lost Mine of Phandelver for the best of these adventures. Lost Mine probably gets it, not because it's flawless, but because it has a greater range, both providing a directed experience while allowing the players agency.
Cheers,
Merric