I've set up a pair of ERA 300 speakers and i'm seen also not an option for balance between left and right.

Also this morning, for no reason, the left ERA 300 stops playing music where je was normally seen in the Sonos app. After unplugging the power the sounds starts to play music again.

I think there are some issues and hopefully, there will be an option for balance soon!

Does the lack of a balance control when they are used as a stereo pair mean they're not technically acting like a traditional stereo pair?

They are definitely a stereo pair, and songs with lear elft and right music play as recorded.  I think that because they are spatial audio speakers Sonos assumes balance control is not needed?

@bockersjv

OuickTune is an auto feature when the mics are turned on. Click the link below. Your assumption regarding balance control may be correct. 
 

https://support.sonos.com/en-us/article/tune-your-era-100-or-era-300-with-trueplay

Edit:

Sonos may also assume that with QuickTuning and Advanced Tuning the balance will be optimized given the type of speakers they are.

I found this on the Sonos site https://www.sonos.com/en-us/guides/era300

"Traditional stereo setups often have an ideal place to sit in a room for the best listening experience, but with Era 300 the sound is sent in all directions so you'll experience rich detail no matter where you’re seated"

Once again the question:

Is quick tune available, if a sub is bonded to an Era single speaker or stereo pair? 
I can‘t find any information about this in the support documents. 
 

Ok, I got an answer from German support team… 

Quick Tune is NOT available for Era speakers if they are bonded to a sub.

:-(