sort of an inverse KVM. and without the ridiculous cost/licensing issues of the official synergy.
An often overlooked feature of synergy is the ability to share clipboards between machines. You can copy a URL from your email and paste it into a browser on another box. Particularly useful if the reason you're doing synergy is to do cross-platform testing and validation.
The thing that made me stop using it was that they punted security to be someone else's problem, so you had to set up some ssh tunnel and be sure to run it only over that. It's not so bad on Linux or OS X, but that's quite a bit of extra work on Windows.
Does barrier take care of authentication or session encryption?
I've only used it between macos and linux machines, so that's easy. An example of the very tiny shell script that I use for VNC-over-SSH to a remote machine.
In which the VNC daemon on the remote machine only listens on its own localhost, and I use ssh to form the tunnel then use the vnc client on my workstation to connect to localhost:5902 to access it.
ssh -v -L 5902:127.0.0.1:5901 -C -N -f -l myusername -i ~/.ssh/my_ssh_id remotehostname.net
echo "localhost port 5902 for the VNC client to remotehostname.net"
I actually think this is better because for a very small open source project like barrier, that might literally be developed by one person, the workload and time/effort to be ABSOLUTELY CERTAIN you've implemented the crypto libraries correctly is a lot of work and worry.
Whereas if you use ssh you can be fairly certain that it's been battle tested by a huge number of people who have a lot more time and resources than yourself.
Nowadays, we have wireguard, so you can create a secure little network to run this sort of thing over much more easily.
Running tailscale (https://tailscale.com/) on each machine you're using, and then using their tailscale private ips with synergy, should be both secure and work painlessly across those three platforms
Since the typical use case for barrier is to have something like two desktop PCs, each outputting to two displays but with no mice or keyboards, and one laptop in the center, where you want to use your laptop's keyboard and trackpad to run everything.
Tailscale has much better UX, so it solves that problem.
In addition, wireguard is just as simple to setup as ssh (again in my experience), can operate over local LAN too, and some people have found it to have better performance than ssh forwarding (such as https://news.ycombinator.com/item?id=21162273).
Other times, it'll refuse to start working again, requiring me to restart one or both computers, with know way to know which is messed up.
If I could guarantee one of the non-subscription ones would work properly, I'd probably pay. But since Barrier is apparently a fork of Synergy, I don't trust it. And most of the others have weird monetization, or I've heard bad things about their customer service... So I don't trust any of them.
In the end, I'm still using Barrier and just dealing with its problems.
The tool I've tried gave poor results with lots of latency.