My opinion: too little too late too big too featureless.
What people wanted was a quick replacement for GIF. They got a bunch of monsters instead. PNG. MNG. JNG. Alpha transparency. 32-bit. Who wanted all that?
"At the time of this writing (1997), the MNG specification has undergone some 31 drafts--almost entirely written by Glenn Randers-Pehrson--and seems fairly close to being frozen" www.libpng.org/pub/png/pnghist.html
I remember back then trying to convince Glenn to add .ECC features like a script (only one pic stored and it could be played several times) but he didn't want to make MNG too complex. Well, maybe he was right, but...
Meanwhile no working MNG creation library in sight for years. PaintShopPro was the first to come out with a working model, but the files, being 24/32 bits, were huge. No way this could go on the internet. And basically only PSP could read those MNGs. Other MNG players would have limited degrees of success playing these back.
The main failure of MNG IMO is that it didn't take bandwidth in account. No real effort was made to save space. And back in the day, playing back images at 24bit was SLOW.
There should have been a MNG8, a direct clone of GIF with just the LZW replaced. But no, they had to make it "perfect". That alpha crap may be perfect but in practice it's a pain. Browsers had a hard time with it, and probably still have. Currently it's basically only used at 2chan to play pranks with PNGs, a pic of a pretty girl thumbnail becomes another pic when viewed in full. Whee.
Meanwhile, the patent on GIF expired. Bye bye MNG.