Static.app is a static-site host: you give it your files and it publishes them. Undraft hosts files too, but hosting is the means rather than the point — it exists so reviewers have a live page to leave feedback on.
Static.app treats hosting as the deliverable. If what you need is somewhere for a static site to live long-term — a real published site rather than a page in review — a dedicated host is the better fit.
A reasonable workflow uses both: review on Undraft while the work is in progress, then publish the result to a dedicated host.