Inefficient and low impedance speakers are IMO/IME a mistake.
All amps make more distortion playing into lower impedance. If you want the amp to sound smoother and more detailed, set it up to drive a higher impedance.
Inefficient speakers need really powerful amps to play properly, and they suffer thermal compression. Really powerful amps that actually sound like real music are not that common and likely pretty expensive, when the whole thing could have been solved without any loss of bandwidth or resolution by simple getting a speaker that's easier to drive.
Speaker cables get far more critical on low impedance speakers too. You can easily spend $$$$s on speaker cables, but if you have a higher impedance speaker the cables simply won't bring as much to the table.
There's truth to this, I find. Obviously efforts have been made by the industry to cover up more or less a progressing deficiency in speaker sensitivity (in tandem with lower impedance) for decades now, as pointed out by John DeVore:
https://www.youtube.com/watch?v=PEcFkSQMc8g
Speaking of difficulty of load though low impedance may be a much lesser issue than what's faced with complex passive cross-overs and their potentially high phase angles. While higher impedance as an isolated phenomenon relieves the amp in a sense, the negation of a passive cross-over, not least a complex one, will have much bigger implications here. Just like with high(er) impedance and its lessening of the importance of cables, the same is offered us going active and the importance here of amps: an important culprit of what make amps sound different lies in the passive cross-overs of speakers, and how well a particular amp is able to remain electrically indifferent to the load presented to it; remove the passive cross-over and amps start sounding somewhat more similar (though not completely similar) looking directly into the specific driver, while seeing their fuller potential much more easily realized.
Active also means being able to direct more power and quality to where its needed. I have 30 watts pure class-A designated to the 111dB sensitive horns, some 2kw Lab.Gruppen to the 100dB sensitive bass cabs, and 500 watts Crown to the 97dB tapped horn subs.
I'm sure many don't feel they're making any outright mistakes with their choices of lower efficiency (/low impedance) speakers, and I'm sure many of them have great sound. Coming down to it, that's all that matters. From my chair though speakers have been way too inefficient for too long, and too small and too expensive at that. I simply don't buy into the "small speakers small room, big speakers big room" mantra that's so often leveled at us. It's an excuse to make us buy generally small(er) speakers and make us feel good about it, when most of us don't have bigger spacings allocated for our audio passion. Truth is though big, pro cinema speakers can sound fantastic in a moderately sized domestic setting, even though they're build to fill auditoriums for hundreds of people. Talk about being preconditioned to a certain way of thinking.
Boiled down the mistake is buying into it all and everything - all this "hifi." Let physics have their say with speakers, be critical of the industry, and keep an open mind - not least a pair of open ears.