I used to be an EU civil servant so I know a bit about this and care very much about it.
Essentially a backstop is a temporary solution that would see the UK stay in the customs union for a limited period after the end of the transition period - onwards from 2020. The idea is that it would work as a temporary customs arrangement whilst a more permanent solution takes shape. It also for the time being would resolve everyone's worst nightmare which is a hard border on the island of Ireland.
The problem is, thus far, no-one has been able to explain what would happen when the temporary backstop ceases to be temporary and more over, removing that backstop would bring us back to step one: how do you maintain a 'borderless' Ireland and indeed how could this be achieved without compromising the integrity of the UK/NI.
The boundary between NI and Ireland runs for miles and miles. It's so easy to traverse that it can never be wholly policed without very stringent and significant physical infrastructure. To build such a behemoth structure would require millions of pounds and security personnel. And that's before you even get to thinking about customs officials. I'll forgo elaborating on the social and political chaos it would most certainly cause.
A border in the Irish sea doesn't mean a flotilla somewhere in the middle of the sea. It means sea ports and airports in both NI and UK would become requisite customs check points for goods, immigration etc. The sea in itself therefore becomes the border. This is pretty much like trying to fuse bones back together using a bandage because whilst the border won't be on the NI/Ireland boundary, a border is a border is a border.
The UK had mooted the idea of using 'smart' technology to facilitate the customs on the NI/Ireland boundary but the problem with that is technology which could do that does not (yet) exist and to get it to a level of sophistication that could manage customs arrangements would take a lot longer to develop than the UK leave date of March 2019, and indeed a lot longer than any transition 2020-period.