I suspect both NI and RoI say they want an open border, but actions and statements since don't support this south of the border. All one hears from there is that the EU demands are all that's on offer, knowing that forces no deal and the border situation still not agreed.
The EU are using the concern to avoid finding a reasonable solution to Brexit. They can agree what they like between themselves on their side of the table but that means little if it is unacceptable on our side. May got nowhere multiple times trying to push the nonsense she'd considered acceptable.
Random checks on goods can be made at the destinations as desired. No one ever catches all smuggling anyway, EU borders aren't immune. Sub-standard goods can be found, were they ever to cross, and the law chase the trail back to catch offenders. It's all just a convenient fuss in order to be difficult.