Bringing a pistol or a pistol cartridge weapon to a gunfight as your primary is a decided poor idea.
In a covert role, this PDW's have a niche. I've worn a Sig552 ina should harness as my "James Bond" gun - into areas that a long gun was frowned on, yet I needed a long gun, under the US Army SPEAR fleece it disappeared yet was pretty accessible if need be.
The issue is when some people (Soldier Systems, etc.) see it as sexy and all of a sudden want to issue it to normal troops as a replacement for a rifle...
Since the rifle, or carbine is too heavy and cumbersome (yes a direct quote by a CF Major EME in Soldier Systems as his justification to get CSS a PDW)