Sure, they are efficient. But without an extremely smooth, warm amp, they make my ears bleed.
I have a pair of Sony SSU-1800's (home audio). They have polydome tweeters/mids. They are so warm that they literally put me to sleep at FULL VOLUME. I'm talking VERY loud. I can't really explain it to be honest.
But setup my Technics SB-440's in my bedroom at the lowest possible volume that I can still tell what I'm listening to, and I'll struggle to sleep. Again, it's hard to explain. It feels like the sound cuts into my ear, and is almost too detailed.
Luckily, I'll eventually have the opportunity to get some tube amplification to power those 440's and soften them up a little bit. They are remarkably accurate and have a fantastic midrange, but they are just too harsh off solid state power.
Sure, this isn't as big of an issue with car audio speakers. But because of my experience with harsh speakers in home audio, metal-composite tweeters are immediately a turn off (sure, there may be some non-harsh metal-composite tweeters out there, but I've never heard 'em).