On PS4, they're working alright. There does seem to be a bug where online doesn't work after x number of hours, so if you get "failed to login" repeatedly or if it says you are logged in but nothing online works, restart the application completely.
With that out the way, it's a ton of fun. So when you're invaded, you run a risk of losing your staff and you can be invaded even when you're not online. When an invader comes in, they can steal staff and resources,
but these resources are unprocessed and online only resources, your mother base stuff will not be affected. But half the reason you have an FOB is resources, so still make sure to try and defend them.
Your staff is a whole different matter. Depending on what platform is invaded, they can take security staff and staff who are assigned to that platform. To avoid this, go to staff management and put your best staff under direct contract. This way, they cannot be stolen.
As for the gameplay itself, infiltrating a high level base is freaking hard even if the other player doesn't come back to defend. It's so rewarding when you pull it off though and the higher the risk, the higher the rewards. If you see high level staff and extract them, you get them anyway even if you lose. BUT, if the enemy retaliates, they can get them back within a certain number of days. Try going full stealth and tranqing guys, but without the crutch of a buddy and a much shorter reflex time, this is much harder than the main game.
Defence is easier, but don't think it's a cakewalk. Try and let your staff find them first, but if they don't, you run the risk of getting them extracted. It's tense when they're never found.
Here's a video of the last minute of a defence I did. You can see I'm frantically searching for this guy that is knocking my guys out and there were 5 minutes of this panicked searching. If they do extract anyone, go retaliate and save them godammit. They're your men!
One thing I would recommend is going to the relationships page. Pick some high ranking guys and support them, hopefully they'll support you back. What happens here is when you're not free, they will come and defend your base if they support you and if you support them, you'll get alerts when they're invaded. You get much better rewards for defending someone else's base, and the satisfaction you did a good deed.
All in all, it's a damn fun mode for me... When the servers work. Even when they're up, it can be frustrating when you can't create a session to defend your base, but the ratio of this happening appears to go down.