I think different frequency range presents its own difficulty. The upper mid/treble region does have its own issues. On the opposite pole, the lower bass has its own unique issues as well. I've designed speakers with both ported and seal designs, and the ported has more "bass", although the seal bass theoretically can extend lower freq., it has a less perceived bass vs. the ported design. On the flip side, the ported has a higher order high pass slope, so it has more group delay vs. the more shallow slop of the seal bass design. For monitor design with smaller woofer, a ported design makes more sense since it has "more bass", at the same time, it is small enough that does not load up the room and may cause room issues. I've used seal bass design for monitor and they just do not have enough bass to justify the more "bass quality".
For three way design, with a larger 8in woofer for example, you could afford to use seal design, but given the same 8in woofer, I could clearly hear the less bass output from the seal design. I think if you have a good cabinet design, a ported 8in woofer will sound just as good and a 8in. woofer is still small enough that will not load up the room and cause issues and that is if the cabinet is well designed.
Most Thiel designs use Passive Radiator. In theory, it has a higher cut-off vs ported, but lower cut-off vs the seal design, so it is somewhat in between, with respect to efficiency and transient response. The ported design has the most bass efficiency but poor transient response, the seal design on the other hands has the least efficiency but the best transient response, and of course the Passive Radiator is somewhat in between.
