Named pipes and tee (or gnu parallel, depending on the problem) make this semantically much clearer. It's so much better than bracket-and-sed-hell spread out over different lines.

Where does sed come into this?

It doesn't necessarily -- I just meant that if you have <(<(...) <(...)) type structures then adding a punctuation-jumble like command in the middle of each subsubshell is a good way to murder readability quickly. Sed, and to a lesser extent, awk, tend to be good examples of tools that (can) use a _lot_ of brackets and symbols...

How does error handling together with this work? Can pipefail catch this or does one explicitly need to ‘wait’ for the background processes and check them there?

Guessing that pipefail cares 0% of which filedescriptors are used and only the exit codes of processes