In many cases, only a fraction of the ICs on a board need to contain boundary-scan in order to test a significant number of nets. Many boundary-scan devices tend to be complex ICs (FPGAs, PLDs, microprocessors, ASICs, etc.) with a large number of I/Os and therefore are capable of providing direct access to a large number of nets. Also, through the use of DIOS modules, you can greatly increase the test coverage. JTAG ProVision's fault coverage feature will tell you the attainable testability and your actual coverage.