It depends how you define keirin.

Inherent flaws:
The aren't as stiff, They weigh more, they allow for no adjustment without serious compromise, they get stuck in steerer tubes...
Real world flaws:
Not many angle choices available, even less with front opening, none for anything other then 26mm bars, alot more expensive for cheap ones if you want to try new postions...

If they were cheaper to make why would they only be found on bottom of the line bikes?