Firstly, as a parent to a 'conventionally' produced IVF child I am unpersuaded by any moral argument against artificially assisted conception.
With that in mind, and after reading the article you linked to, it would appear that there are some scientists saying mitochondrial replacement is safe and some that are saying it might not be safe. It was ever thus with any new scientific development.
As maintaining the status quo will definitely end with deformed cells and disabled / dead babies, whereas the new technology MAY result in learning difficulties, there's no real reason to abandon the technology yet, rather that its a case of a bit more testing being needed.
I just don't understand these "risks". Humans from all different races, blood groups, hair colours, etc., have been interbreeding for hundreds of years now. Have any "mismatches" in mitochondrial DNA been found so far?
Either way, the three scientists in the quoted article are supportive of the procedure, but would like more animal studies to take place first.
I'm firmly in support of the procedure. Humans have negated the evolutionary process in the last 100 years due to improvements in social care & medicine. We will need to take control of our own genetic health in the future, and techniques like this will be essential.