I have 2 Roku 4661R devices, and I am wondering if via the USB port it will support devices formatted with the APFS File System. I have been using devices formatted with the HFS+ File System, but I am using the latest version of the Mac OS (Mojave), which uses the APFS File System.
I also have 2 of those Roku Ultra devices, and also for my 2 Macs, run the latest version of Catalina, which requires the device/partition it is running on to be formatted as APFS. But other devices, partitions, etc, can be formatted as HFS+, EXFat, etc. Of course the preferable "other one" is HFS+.
I have 3 Samsung external SSDs. All 3 have 3 partitions. 2 of them have 2 partitions formatted as APFS for my SuperDuper! backups, and the third partition is formatted as HFS+, and it contains various files, folders, etc., including movies and TV series. The third SSD contains an APFS partition for (future) Big Sur testing, and 2 other partitions formatted as HFS+, one containing a lot of movies, and the other containing a lot of YV series. For any of those SSDs, via a USB connector plugged into either Roku Ultra, I can watch a movie or a TV series contained on those HFS+ partitions.