I think it is good to know, that it does not raise io error, so it is not as reliable as an SSD or HDD. It is better to backup data to a HDD frequently for example if you run a database from SD card.

According to this: superuser.com/a/168126/105936 it is not such a great idea to put /var/tmp to memory. Maybe you confused it with /tmp.

In my case the HDD spins down after 10 mins idle, when the server is running (odroid), but when it is unplugged or the server does not run, then it spins indefinitely. What happens by your external HDD when you turn off the rpi?

A proper SD card like this: bhphotovideo.com/c/product/1082349-REG/… wears out after 100TB of writing. This is not an issue, only if you use something very cheap. Every sandisk sd card uses wear leveling since 2008 (if I remember well), and I guess other top manufacturers use it too.