It's also an enshrined right in the Good Friday/Belfast Agreement that the citizens of Northern Island can be both Irish and British, and as I understand it the lack of a border between the two has been the symbolic embodiment of that right. So for a good subset of the Northern Irish, to at all reinstate that border is a symbolic removal or dirtying of what to them is a hard-won right.
So you've got both practical concerns, and the more ideological ones about Irish/Britishness that Good Friday agreement has managed to keep at least somewhat swept under the rug. Which is where some of the concerns about a reemergence of the Troubles come from even if it's a softer-but-visible border. Northern Island has been more peaceful since the signing of the Good Friday Agreement, but we can't ignore that it's not exactly been
completely peaceful.
Got to be careful not to pour fuel on a fire and turn people from legitimate political republicanism and to terrorism. If the UK government allow the legitimate achievements from negotiation and diplomacy so far to be so easily perceived as being undermined, and worse being undermined not by the will of the Northern Irish but by Westminister overriding the will of the Northern Irish, it risks pushing the people already on the edges closer to attempting those 'alternative approaches'. Quickest way to create extremists is to fail to address the concerns of moderates.
(Of course, if you add extra border checks leaving Northern Ireland and into Britain then you have the second verse same as the first, but with Unionists).