Why would anyone want class AB amp when class A always sounds better ?

Are class A amplifiers significantly harder to engineer, or they are about the same but only bring some of the well-known things (heat) with them simply because of class?

Also, how does efficiency of class-A amplifiers compare with other classes (ballpark figures)?

They are not harder to engineer. Its arguable that they are a bit easier, although you have to be more careful about heat issues and the power supply has to be able to support the output section being on all the time, something that you don't get with AB amps.

Of course class A is the least efficient of all classes of operation. The advantage is the output devices are often biased at the most linear portion of their curve. So if all goes well, they will make the least distortion.

You see. Ralph too says yes, class A is the way to go.
So you guys dump your second rate stuff and get real. But whatever you do I certainly will.

As a casual listening “semi-audiophile,” my journey took a delightful turn when I started digging through the story of the early Crown SS pro audio amps. I ended up scoring two PS 200s in decent shape for under $200 each, sent them back to them back to the Midwest for refreshing and thoroughly enjoyed building two different systems...a 6U rack system with modern processing and vintage Cerwin Vega/Jensen sub speaker combo...and the other a “rat rod” rig with nothing but an unpowered Shitt preamp and BT2 Bluetooth DAC with modern REL T5I sub and Onkyo towers both running off high output.

We don’t play much volume around our home and cottage, so these venerable refurbished 90W Class A/AB/A+AB Crowns, both removed from churches, run in crisp Class A most of the time and present a surprising snd pleasing sound stage. Great fun and great value in the final analysis, FWIW

...and yes, they help heat our respective living rooms :)