On one side I do dumbpipe listen > file.txt.
On the other dumbpipe connect ... < file.txt.
There is no indication that file transfer finished, I expected that dumbpipe connect will close once the file is transferred.
On the receiver side would also be nice to have an option to close after receiving a file, afaik ncat -k option is the reverse of this.

Generally the use case of transferring a single large file is not very well covered.