Is a 20 disk raidz3 really a bad idea, for media archiving use cases?

I am archiving a vast amount of media files that are rarely accessed. I'm writing large sequential files, at peaks of about 100MB/s.

I want to maximise storage space primarily; I have 20x 18TB HDDs.

I've been told that large (e.g. 20 disk) vdevs are bad because resilvers will take a very long time, which creates higher risk of pool failure. How bad of an idea is this?