There is only two of you, and you may have differing priorities. You might be looking at the place long term, wanting to upgrade mechanicals/landscaping/roofing etc, the other person might be looking to get out in the next year or 3 to get more space in the burbs so just wants to band-aid everything.

Things like putting in solar or doing electrical work to get an electric car charger are two areas I can see where things could be tricky in a two person condo if you aren't on the same page.

Another potential area of concern: Your neighbor can just decide to stop paying the HOA fees--and short of taking them to court, you have very little recourse as to how to collect. They could also decide that those roof repairs aren't their problem, since they're on the lower floor--again, what can you do short of taking them to court? There's no Condo Association or Board to enforce the bylaws or provisions of the Master Deed. It's just you and your neighbor.

That said, you could also end up with great neighbors who follow the agreed-upon rules.