The issue with the ARM based NAS models is the lack of supported 3rd party software, be it from Synology or QNAP. You tend to get about half of the available options, which means that you might not be able to use some of the programs you thought you'd be able to run on your NAS.
The other problem I have with the ARM based models is the inability to add more RAM. This is quite fiddly even on the x86 models, but at least it's an option. 512MB of RAM is just stingy imho.