NI was not caused by Brexit, it was caused by the reaction to Brexit by the EU, an organisation that would not sort out their own border checks away from the Irish border, because it served their purpose to cause trouble, claim their required checks were the responsibility of the UK not them, and blame the UK for their failure into the bargain. Some folk seem to have fallen for that ploy.
As for border issues, it may not be directly a Brexit issue but since Brexit proved necessary but insufficient to return control of our borders to us it was up to parliament to withdraw from whatever was preventing us applying sensible action to solve the issue. Neither the present government, nor the present main opposition, has shown any sign of trying to progress that; the most we have heard are individuals talking the talk but seemingly prevented from walking the walk.