I don't think there's any such thing in the universe as "unlimited." ;)
But seriously, if you're using the phone on Wi-Fi, which is where the vast majority of people use their phones, then the only cap is the one on your broadband connection (usually at least 200 GB). I would suspect that for greater than 97% of cell phone subscribers, Republic's data limits are not going to be an issue.
And frankly, I think it's high time a new carrier disrupted the AT&T/Verizon/Sprint/T-Mobile oligarchy.
If I remember right, the way it worked out back in the days of AT&T's original unlimited iPhone service plan was that basically everyone was paying higher prices in order to subsidize a very small number of YouTube addicts whose 3G bandwidth usage was larger by an order of magnitude or two. Keeping that kind of user off the network would be a key part of making prices reasonable for everyone else.
Like every carrier, there is no such thing as "unlimited".