The only way to see what is happening is UART console. LEDs are controlled from software, so they can't tell you much.
You can't put whole rootfs on FAT partition (at least without some very strong magic to emulate file permissions and owner information). These "small PCs" are using u-boot which does have ext4 filesystem drivers, well, unless you build your image on Debian Stretch (testing) or sid, which uses ext4 flags not supported by u-boot.
Yes. Compared to Raspberry Pi, these boards are not perfect for novice users, and also most things are already documented (i.e. here)
I wouldn't call this a trivial situation. If you search through this forum, you can see that most of early boot problems are related to either SD card problems (including using wrong/incomplete procedure for writing an image) or power supply problems (or interference from connected USB peripherials), but it may take ages and insane efforts to force users to admit that problem is on theirs side.
These images are designed to work out of the box, you don't need to touch anything related to partition table
