How do I run unison between Ubuntu and CentOS 7

Per @Freddy's comment, the key is to compile unison from source on CentOS:

yum install ocaml ocaml-camlp4-devel ctags ctags-etags

cd ~
wget http://www.seas.upenn.edu/~bcpierce/unison//download/releases/stable/unison-2.48.4.tar.gz
tar xvfz unison-2.48.4.tar.gz
cd src
make

sudo cp -v unison /usr/local/sbin/
sudo cp -v unison /usr/bin/

cd ~
rm -fr src

Then you will be able to run unison:

On the local server, go to the desired location, then test it with

unison -testServer . ssh://myuser@myremotehost//path/to/data/

To do the real thing, simply remove -testServer from above. It's also a good idea to tmux before doing so.