Sorry maybe I wasn't clear with the QNAP 464 and the second PCI-E slot. It should have worked but mine, and many others was faulty, due to some sort of hardware failure. The retailer and QNAP were quite prepared to swap it for another unit but by that point I'd lost faith and was n't willing to take a risk of another faulty unit or it failing further down the line. So no con just a faulty batch probably.
Re Synology - yeah the focus of their prosumer/SOHO units seems to definitely be moving away from Intel and onto AMD, which if you're just doing storage/file serving is fine and understandable but anything home media server related is not good at all, like wise they're attempts to lock you into overpriced proprietary hardrives, SSD's and RAM is just a joke.
So yep my 920+ will be my last Synology NAS till the hardware & company direction changes somewhat.