That's why I'm not a fan of the 3-4.
If you commit your OLB's to rush then the flat is open. If you split the ILB's to compensate, the center of the field is vulnerable.
If you don't rush the OLB's your DL is outnumbered and you have no pass rush.
Unless you have extraordinary ILB's you'll have holes.
I suspect that's why the safety-backer has become so popular. He has speed and tackling ability.