The "land border" we currently have is, presumably, the Channel Tunnel and (at least in theory) shouldn't be so hard to monitor (the practice is, perhaps, different). On the other hand, a split UK would introduce a land border measured in hundreds of miles, rather than a handful of metres.
All these issues are up in the air, of course, and there's no reason to assume a worst-case scenario, but it's still worth bearing in mind that our potential split from teh EU is never going to be so neat as people would like.