Hello Stuart,
@Themevision please ensure stability of new versions before rolling them out, we are running businesses and this level of risk is not professionally acceptable.
First of all, i have myself tested many many times everything before i have released new update, and i can be 98% sure that every migration must be finished without any trouble or error or anything else.
200 clients did migration without any issue, however there could be situations like yours but not because we are unprofessional like you are stating but because you have too much plugins installed or something else could be issue which is doing a conflict when migration process is starting.
I hope you read my e-mail where i have send all clients instructions how they can migrate their options under customizer and there you would also find that i have stated the importance of theme backup before migration process is starting.
Anyway as professional developer i can just offer you to allow me to do migration process on your website & if you are ready to allow me that you can contact me directly at: kdonet @ gmail.com
Best Regards