How is SD-WAN and SDA accommodated in Aviatrix?

Aviatrix integrates nicely with pretty much any SD-WAN provider. You can take a look here for a very common deployment model.
https://docs.aviatrix.com/HowTos/transitvpc_designs.html?highlight=SD-WAN#sd-wan-integration

How does CloudWAN take the optimal route?

We leverage AWS Global Accelerator in AWS and Azure Anycast routing to connect the branch to cloud. With this, the branch will connect to the closest point of entry into that cloud provider's backbone. 

Any plans to extend CloudWAN functionality to other SD-WAN solutions (VeloCloud, Versa, Meraki, etc.)?

We have customers integrated and are currently using SD-WAN and Aviatrix together.  We are working with teams in WWT with a joint solution launch of Aviatrix + SD-WAN Better Together story.  

Does Aviatrix have Enterprise customers leveraging CloudWAN today? Were there any specific challenges with the deployment you could speak to? 

Yes, enterprise customers are leveraging it. We developed this feature for a very large hospitality chain with thousands of branches with Cisco routes. Enterprises do not want to rip and replace their Cisco gear with SD-WAN gear.  In some cases SD-WAN is an overkill. 

Challenges: Not really because if you look at it, it is very seamless without much complexity involved. I can tell you that these enterprises are asking for more features and options in our CloudWAN and we are working on improving on the feature set. For example look here: https://docs.aviatrix.com/HowTos/UCC_Release_Notes.html