The UI comes at a time when most of the CS's have most of their land improved anyhow (I play on the "plus" maps), and finding an unimproved plot of land that I then have to send my worker to is a bit tedious.
You don't need an unimproved plot to build the UI. You can simply replace any improvements for it for as long as it's coastal and doesn't have a resource on it.
Btw, Arabia disappeared again.
Arabia - 23
Aztec - 17
Babylon - 7
China - 22
England - 15
Ethiopia - 16
Germany -22
Inca - 19
Korea - 24
Maya - 24
Poland - 25
Portugal - 13
Siam - 7
Zulu - 19
What I do like about Portugal is that their uniques synergize with each other, albeit in a different way. Naus are the fastest caravels in the game, not counting England's UA. This makes them easy to spot for good trade route candidates later on in the game, which is their UA's specialty. IIRC, the UI also affects their UA's gold income. True, you could ally the City States instead using the gold income you're generating, but if the CSs are bought by, say, Greece, you'll still gain the luxuries from the CSs, even at war. As I said before, Portugal done right will be the toughest civ to get unhappiness with.
Zulu. Not really a bad civ and actually, just like Portugal, the uniques synergizes very well. Unfortunately, I would also call it a crippling overspecialization. The lower maintenance helps with a bit of budget management, but then, they have no other means of generating income. Besides, even if you went peaceful, you really only need a handful of units to defend yourself anyway. Not only that, even if you do go for warmongering, you're still limited by unit supply, thus it does force you to build new cities and/or capture a few others, which brings us to another problem: unhappiness. The UU and UB are, by all means, very good but, as I said, too specialized. At the very least, Portugal's money can be used for city improvements, research agreements, diplomatic bribery and much more. More impis? Not so much.
Edit: Forgot to subtract Zulu.