Wondering if someone could help explain the m.2 nvme vs sata situation with a motherboard in layman’s terms?

I’m looking at purchasing the b450m mortar max motherboard, from what I can see it has two m.2 slots one of which (M2_1) that supports sata and nvme and the other (m2_2) that supports just nvme. From what I understand if I install the m.2 in the first slot it should be fine, but installing it in the second removes the option to use the pci_e4 (to install a second gpu?)

So my main question is what is the draw back from using either slot? Or would I be ok to use the nvme m.2 in slot m2_1?