You are absolutely right in saying that the issues that you read about ReFS should scare you. But there is good news. You might have read the word from Gostev this weekend and saw that the necessary patch for these problems is available (again). https://support.microsoft.com/en-us/hel ... -kb4088787
There is still an issue with memory consumption and they are working on a patch for that but this is not the case for everyone and is more for extremely large repositories. We do think you should look into formatting the ReFS volume with 64k instead of 4k.
For your BDRS on site, will there be synthetic full backups? In that case it is certainly worth it in investigating and formatting with ReFS and see the benefits. The same requirement obviously stands from above. The biggest benefit is that block cloning technology that we use. Merges will be much faster and even use less storage because of the linking and not actually copying data anymore.
Considering the both sizes (18TB and 12TB) I wouldn't worry too much and with the patch it is safe to use.
Hope it helps