One of my bubs was a morula that I was told wouldn't make it to freeze. I thought it deserved a chance and they agreed to transfer 2 (I am guessing they agreed because they thought it had no chance). Both my clinics wouldn't freeze morulas. Shame. It seems like they might actually be the normal ones.
The interesting thing about slow embies is back when it was legal the 'cure' was to transfer some mitochnodria from another womans egg. Now if the eggs were failing to progress because they were chromosomally abnormal then producing embies from them would be disastrous. That's not what happened though. There were higher rates of Turners syndrome that the norm but not as high as what you would expect. So obviously not all slow eggs are slow because of chromome 'mistakes'. They just lack energy.
You may have read that the UK legalised this procedure earlier this year... but only for those women who have mitochnodrial disease.