Canada

Mainly just that we'll need to align stuff for like a decade before we can do it. Using the Euro and joining the Schengen is non-optional, although it's one of those EU things where there's a lot of chaos in actually enforcing that. Speaking of the Schengen, that would mean our own trade deals with the US are definitely dead and the border has to get much more serious, since it's now the border between the US and every country in Europe.

It would also make Kosovo and Bosnia butthurt if we got in first, so there's a chance they'd just deny it on geographical grounds - we could qualify as substantially European, but only could.

All in all, I think it's a great aspirational goal, at least, because "substantially align regulations on control of tomato leaf miner pests" isn't quite so catchy.

The EU and US standards are very different and products for one can't necessarily be shipped to the other and vice versa. Examples for food include massive differences for colourings, preservatives and the like. Europe will not accept chicken washed in chlorinated water or bright froot loops. Health traffic lights are also going to be different.

While it's possible to have 2 production lines, 1 for each economic zone, that's expensive for producers and shippers.