Secret service has very different criteria than a SWAT team or elite military unit. They want very controllable fire with high volume at a target or targets that are likely quite close. They have counter sniper guys as well, but that's not the job of the ones standing around the big man. And at 900 rounds per minute, you're not so worried about what one 5.7x28 projectile will do.
While I'm no fan of the round, I think it is more effective than the .380 cal. Ingram M-11's they've used in the past, which were also very difficult to control with a 1,200 RPM cyclic rate in a 3.5 pound pistol. They empty the 32 round mags in a second and a half, and will string vertically for all but the strongest and most proficient user. The P-90 has greater range, greater penetration and is much more controllable, at the cost of a bit larger dimensions and an extra 2 pounds.
That said, for a PDW role, I'd prefer my semi-auto 5.56mm PLR-16 to either of the above mentioned guns. It's as light as the M-11, much more powerful than the 5.7x28, and more easily concealed than a P-90. With 20 round mags, the thing can be carried under a jacket no problem. And they're very controllable in rapid fire.