The Intel X299-chipset is not radically different in comparison with the X299-chipset, which has lasted two generations (Haswell-E and Broadwell-E). That said, the platform is more modern in a few regards. It is a public secret that the X299-chipset is very closely related to the Z270-chipset for mainstream systems. For example, X299 also supports Intel Optane-memory, among other things.
Compared with the X299-predecessor the amount of internal lanes for all kinds of connections has been increased quite a bit. In total there are 30 I/O-lanes and a few of them have a set destination. The chipset offers ten USB 3.0-connectors and eight SATA600-connectors. Note that the latter is less when compared with X99. Other lanes can be used by motherboard manufacturers for, for example, extra PCIe-slots, M.2-slots or additional controller chips.
In order to supply all those connectors with enough bandwidth, the internal DMI-link between the processor and chipset is upgraded to PCI-Express 3.0 x4. This means that the maximum available bandwidth is 4 GB/s. A quick calculation tells us that this is nowhere near enough to use all USB-connectors, SATA-connectors and other interfaces at the same time at full speed, but this scenario is not very likely.
More or less lanes?
Connectivity via the chipset is great, but the devices with the biggest bandwidth hunger are usually directly connected to the processor. We are of course talking about graphics cards, but also about M.2-SSDs. If you have read our review of the Skylake-X and Kaby Lake-X processors, which are meant for the X299-platform, you know that there are processors available with three different amounts of PCIe-lanes. The Core i5 7640X and i7 7740X only have 16 lanes, the Core i7 7800X and 7820X have 28 and the Core i9 7900X and up have the full 44 lanes.
In short, the available amount of lanes for functionality on the motherboard varies wildly, depending on which processor you choose to use. This poses difficult choices for motherboard manufacturers. Per type of processor they have to determine what components do and do not stay enabled. The models with 28 lanes often receive less bandwidth for the PCI-Express-slots. With the 16-lane processors a lot of the functionality has to be turned off, which in itself is basically reason enough to stay away from these Kaby Lake-X CPUs.