Storage Spaces Direct detect and change Resiliency Mode

In order to change the resiliency settings of your existing volumes you have to recreate the volumes/pools with new resiliency setting and migrate the data. S2D only re-balances the data between newly added hosts keeping the resiliency setting intact.

S2D does not have LRC within physical hosts which means you are basically running on top of some kind of RAID0 in each host. That is applicable to both 2-node and 3-node S2D clusters.

For smaller deployments (especially 2-node clusters) I would recommend using alternative solutions like HPE StorVirtual https://h20392.www2.hpe.com/portal/swdepot/displayProductInfo.do?productNumber=VSA1TB-S or StarWind VSAN https://www.starwindsoftware.com/starwind-virtual-san-free which do exactly the same what S2D does but are capable of working on top of internal hardware RAID array keeping data locality and consistency within each host. This approach is called Grid Architecture https://www.starwindsoftware.com/grid-architecture-page and is way more beneficial for small-sized clusters.

1) There are no automatic rebuilds with S2D, every time you add new node you have to re-create the pools with the newer resiliency option and migrate your data.

2) S2D has horrible resilience with anything below 4 nodes Microsoft initially were planning to put into production. 3 is waaaay better than 2 but still can't match any other mature SDS on the market: it can't survive double faults.

EDIT: That's true for 3-node configuration OP was asking about, 4 and more S2D nodes with 3-way mirror can survive double faults.