When Server Side rendering is disabled via config (ReactSiteConfiguration.DisableServerSideRendering()), components that are not explicitly marked as client-only throws an error to Javascript console, similar to this:
Warning: Expected server HTML to contain a matching <div> in <div>.
The possible case is that while rendering JavaScript for ReactComponents, it is checked only if the component is ClientOnly. If it's not , hydrate is used instead of render.
I think that hydrate should be used only if component isn't client only and server-side rendering was not disabled via config.