Portugal is a civ with which you must trade, and trading works better on the higher levels. Most players will play on a moderate level, and don't check their trade options every turn.
This perhaps goes to some extent for the whole expansionist trait. It gives you earlier contacts, but if you don't use those contacts, yeah...
I would rather encourage players to use that expansionist trait instead of saying the trait isn't good. As soon as you meet another civ, see what they've got, and make sure you're updating your knowledge every turn. Playing with a helper program like CivAssist is a must, otherwise you lose track of your options.
It's great to be able to snap up some early slaves, and the expansionist trait offers the best chances to do that. Scouting to reveal the map is indeed something every civ can do, but to get a trade advantage specialized scouts or curraghs are indispensable. To meet other civs before they've met each other is very important for making the best tech deals. On higher levels warriors are too slow, but I must admit that on lower levels they might still be fine, as the AI is slower.
Early wars area speciality of expansionist civs, because of their diplomatic advantage. Again, if you're not using diplomacy much, then you will miss out, but expansionist civs will have better options for alliances, and are more capable of tactics of divide and rule.
I'm somewhat biased towards Portugal, as it's my favourite civ. I don't disagree if someone says their traits are not complementing each other very well. Portugal is probably best on a fairly large map size, where the mileage of both scouts and boats can be exploited. Pangaea takes something away from the boats, and archipelago takes something away from the scouts, so continents suits Portugal probably best.