The old thread was closed already, but it may be worth mentioning that mounting rclone unions directly can be very slow when listing files due to lack of directory polling for everyday use and Animosity's recommendation to use mergerfs instead is very helpful.
tldr; for everyday use mergerfs on mounted remotes seems to be better than mounting rclone union directly
This may be completely wrong, but it seems to me that mergerfs uses mounted remotes ("branches") directly, so properties like polling directory cache will be preserved on remotes that support it (Google Drive for example), which makes it look like the mounted remote and the union "shares" directory cache.
Because mounted rclone union don't support polling you need to evict directory cache on a regular basis, which adds a lot of HEAD requests. More importantly though stat syscalls were really slow, which slowed down a lot of programs (fsearch, zsh TAB completion).