AFAIK, it should be compatible with YAML to intelligently interpret it’s content. It is just inconvenient to copy links from there manually or use other application to open them when Obsidian is here.
Related question: Will links in front-matter be reliably updated when target file is renamed? Purpose: I use yaml block to store/track context information for pieces of text referenced (transcluded) in other places as “block”.
Update: Links in front-matter do not get updated when file is renamed. (observed in Obsidian v0.9.4)
Related to Markdown links are not clickable in the edit mode
Given that the YAML is being reserved for plugin use, I’d hesitate to rely on it. When that decision was announced dismantled everything I had already in YAML to avoid any possible disruption in the future.