Well if you don't have border controls of any kind, then people can also move freely between the NI and Ireland. Meaning, just like goods, EU citizens from Romania, Poland etc. could come into Ireland without visa, drive across to NI and then on into the rest of the UK.
Now if you do decide to have border controls of any kind, electronic or physical (thus avoiding the above problem as well as the tariff issues from my previous post), then you are breaking the GFA which specifically forbids it.
Whilst I suggest you go and read a summary of the GFA in relation to the various cross-border agreements underpinning it, and how some of these are incompatible with a hard border, I will simply give you one example of the effects:
The Good Friday Agreement provides that the people of Northern Ireland may choose to have Irish citizenship or British citizenship, or both. Those born in Northern Ireland (and their spouses and children) are thus entitled (in the case of spouses after a period of residence) to both UK and Irish passports, although in practice many have only Irish, or only UK, passports.
Over time, reciprocal rights for UK and Irish nationals (some of which are legally enforceable) now include the right to enter and reside in the UK and Ireland, the right to work, the right to study, access to social welfare entitlements and benefits and access to health services, and the right to vote in local and parliamentary elections.
When it was concluded, the GFA clearly presupposed that both Ireland and the United Kingdom would both be members of the European Union — no one even contemplated the idea that one of them might leave the EU, a view endorsed by the UK Supreme Court in Miller. This significantly affected the way in which rights in the GFA were dealt with.
This, in turn, affected the framework of the Northern Ireland Act 1998 and, indirectly, the wider UK devolution settlement.
In particular, rights deriving from the EU provided one element of the common understanding of rights shared between the UK and Ireland (the other important dimension being the ECHR).
In addition, there is a wide range of rights afforded to people in Northern Ireland directly under EU law. These include the European Single MarketÂ’s four freedoms (free movement of goods, capital, services, and people), and rights such as EU equality law, that together with domestic anti-discrimination law, partially meet obligations under the GFA.
Ireland and Northern Ireland are both bound by the
EU Charter of Fundamental Rights, for example, when government implements EU law.
Ireland and Northern Ireland are both subject to the fundamental rights jurisprudence of the Court of Justice of the European Union (the CJEUÂ’), part of EU law’s general principles.
In short, several of the rights and entitlements that are provided for directly or indirectly in the GFA are themselves either directly or indirectly underpinned by EU law, and its system of effective remedies
some extracts taken from : https://medium.com/brexit-briefings/the-good-friday-agreement-brexit-and-rights-1c0af667440b
Hope that's no too complicated for you to understand. 'Cause it's far too complicated for the likes of Boris Johnson, Theresa May et al to understand!
