From that link you referenced.
"(5) The MXL 990 was disappointing. Sometimes it sounded awfully midrangey, other times barely to somewhat usable. It wasn’t used on any mixes because, even when usable, it didn’t add anything to the blend that another mic didn’t do better. An odd thing that I noticed about this mic: For the 990, a recorded signal at any level (say, -6 dB) sounded quieter/weaker/thinner than any of the other mics outputting the same level/signal. At this point, this is the last mic I would use for distance micing. It too often reminded me of the tone that you used to get from those built-in condensers in boom-boxes."
Being mid range focused is why the 990 works well on electric guitars. Most of the disappointment with this mic comes from people that try to use it as a vocal mic. It looks like the type of LDC you would use on vocals and its cheap. A lot of newbies get sucked into thinking it will work for a primary vocal mic. It doesn't. If you want another flavor for guitar layers, it works great. Actually on background vocals that need to sit back in the mix it also works. In this example, they were used for ambient recording, frankly I agree, they would suck for this. They require a high volume source to make them work and are just not sensitive enough for subtle sounds.