There are some incompatible components.

Some protocols depend on IPSec to secure itself.
Mobile-ip, IP address renumbering, etc.
But there are some contradictions between IPSec and these protocols.
Because they are designed individually in the designing stage.

Some technologies are incompatible with IPSec.
NAT, filtering, diffserv

We have to consider the relationship to other components.
They are discussing at IETF.