I have been considering the same thing, but I have a feeling that this will not be relevant for very many before we see stx boards with full pci-e slot.. If that will ever happen I have no idea.
The lack of a full pci-e slot shouldn't matter for this form factor, because you can use one of these M.2 NGFF -> PCIe 4x adapters:
P4SM2 (PCIe X4 to M.2 Adapter)
PCIe 4x throughput is even enough for a 1080, without impacting performance. Since only 1070's are available in a compact form factor this wouldn't be a worry.
I think a bigger concern would be the lack of available STX boards.