I may be late for this but I think, this would be good if we have support for few common image formats so that it can run on any filesystem and not limiting ourself to storage that are not scalable.
They are better managed and gives better performance and reliability in case of storage issue or disaster instead on counting on on single storage box.
For example - I am using scale-out storage based on Moosefs and these are very stable and provides good performance when scaled. They have atomic snapshot feature which can also be used.
I would suggest to look at Moosefs, for more information, its better than btrfs in terms of performance and reliability.
But if we have a image format more common or without limitation then we can use any underlying filesystem for urbackup and it would do great.
I have seen some have requested for vhdx format but that too have limitation of 64TB which is still good compared to 2TB on vhd.
The devloper can decide which should go right may be vhdx, raw or any other, Though this is just my suggestion.
However .raw format will work with most of the hypervisors and gives added advantage to other developers to automate things at hypervisor level too in case of disaster.