In a word ... yes!.
USB and SSD are not, even individually, devices I would even consider implementing on any server let alone a Filemaker server.
They will work, but performance and reliability will be the weak spots. A better solution would be a Mac mini server (without the OSX Server code running) with it's dual hdd mirrored in raid1 and it's memory maxed out. Even better would be a Thunderbolt disk enclosure with fast enterprise HDD configured in a suitable raid configuration and an additional disk for backups.
In my own case I run FMS13 on an Aluminium Mac Pro with boot drive, Dual mirrored HDD for Databases and a single drive for Backups. I have a Mac Mini with a pegesus Thunderbolt enclosure ready to go should the need arise. It depends on how highly your "customer" values the solution you provide, it's latency as it grows and how they will deal with a fall over once it happens. If it is not mission critical and they can live with the possibility of loosing their data ... then sure, they can travel the path they have set. I for one would not be advising it or assisting them if that were the case. Sometimes people are just "too smart" for their own good.