I am in the USA, so I hesitate to offer my opinion. But so what, I offer uninformed opinions all the time.
From my perspective and understanding it appears that a hard border between the Republic of Ireland and Northern Ireland would inevitably stir up the issues, and possibly the violence, associated with the Troubles. The practical problems would be enough to create strong anger in the Irish population and likely damage their economy. The way many Tories have publicly minimized these issues hasn't helped.
But more so, having both the Republic of Ireland and Northern Ireland in the EU, by producing a very soft political border, significantly toned down the enormously sensitive issue of Irish unity. It made the political division between the two Irelands less part of peoples' day to day experience. Those who wished to could even pretend that it was a step toward the ultimate unification of Ireland without having to actually do so and thereby avoding to a strong counter-response from Northern Ireland. But a hard border would reverse this, and imposed by the UK, would surely be viewed by those in the Republic as yet another a slap in their face. It would highlight the continued separateness of Northern Ireland and in a confrontational way that would be evident every day for those seeking to cross between the two countries for personal or economic reasons.
I see this as an enormous risk for violence, with the most radical arms of the IRA being the only ones who will benefit. IMO this is why a backstop is crucial.
Are my concerns shared or stupid?