In addition to the ability to adapt M.2 slots into PCIe via an inexpensive adapter, there is a growing ecosystem of M.2 cards for most of the things you'd use a PCIe card for. USB controllers, SATA controllers, Ethernet controllers, FPGAs, even display adapters.
It would not limit you to just SSDs, the m.2 connector on the rk3588 devices generally exposes 4 pcie lanes and an adapter to make this a full pcie slot is already available. So really an m.2 nvme connector is the best of both, you can turn it into any other pcie connector you want via an adapter, but you can just put an nvme ssd directly on the board without a hat.
