The solution is easily scalable.

It is scalable, but only beyond two nodes. If I go for two nodes it's not scalable. I need to build a complete cluster from the beginning if I'm going for two nodes.

The only challenge with scalability is that after you add hosts to the cluster, you'll want to create a new volume and live-migrate your workloads to it, then delete your old volume. Hopefully, this can be automated with the "optimize-volume" in the future.

This is a scalable solution. You can go from one note all the way to 16 in one cluster. Beyond a single cluster you can use a technology called Cluster Set which allows you to group the cluster together.

Microsoft Storage Spaces is very scalable. You can go up to 16 nodes in one cluster. Then you can group the clusters together using something called cluster sets. You can build clusters as you want using various generations of hardware. So scalability-wise this product is amazing. And also you can add drives into existing notes and also on top of that you can add directly-attached storage to nodes that can expand just the storage without increasing the compute.  

So the scalability is some of the architectural advantage of this solution.  

I would say that it is scalable.