Armaldo is really weird. On one hand it is some kind of pokémon that is supposed to live in the ocean, while at the same time it actually has a water weakness. In Sandstorm in can actually be quite defensive, whilst in rain it can go offensive with Swift Swim. It isn't a great pokémon, especially bulky waters just walk all over it. If it relies on rain support it goes down even faster to bulky waters.
It gained a few tricks since 3rd gen, like X-scissor, Swift Swim, and Stone Edge, but when it comes down to it, it still is a very slow pokémon with barely any resistances and quite bad Sp.Def. Without Sandstorm or Swift Swim those weaknesses still bring about its downfall, and in that sense it is still the same as in 3rd gen. Just too slow and too easy to kill with Surf to really take over the world.
Armaldo still is a cool pokémon though, due to his weird love-hate relation with water. I'm pretty sure that Armaldo is the only pokémon (apart from Anorith) that is weak to water while getting Swift Swim.
Master balls are the cheapest thing ever invented. It's more fun catching everything with balls that can fail. 'caught Latios in the wild without master ball ftw'