The introduction of Skylake made M.2 widely available on motherboards, with at least one slot per motherboard. This generation each motherboard features two connections for PCI-Express SSDs. This shouldn't come as a surprise, since the biggest change in the Kaby Lake chipset, the increased amount of lanes, was meant for this purpose.
All boards in the test have six SATA600 ports, while the higher positioned ASRock Z270M Extreme4 and ASUS Strix Z270G Gaming have two M.2 slots. Only the Gigabyte and MSI boards have an extra PCI-Express x16 slot that can be used for storage (or CrossFire). Apart from that the boards are comparable to ATX boards in terms of storage functionality.
|ASRock Z270M Extreme4||6||2|
|ASUS Strix Z270G Gaming||6||2|
|MSI Z270M Mortar||6||1|
Due to the four extra lanes from the Kaby Lake chipsets there are less conflicts due to shared lanes, however there are few motherboards that allow you to use all slots and ports without any conflicts. Since Micro-ATX boards have a limited amount of bandwidth, the number of conflicts is rather limited however. In practice this means that on these models SATA600 ports are disabled when the M.2 slots are used.
The ASUS Strix Z270G Gaming has an M.2 slot that can be used without losing any functionality, while the second slot can be used at the cost of a single SATA600 port. You will lose one SATA600 port for each M.2 slot you use on the ASRock Z270M Extreme4. It's the same story with the MSI board, however Gigabyte has done quite a good job here: you lose no functionality at all when using M.2 slots.