The question has no meaning without specifying the languages the learner already knows. There is no such thing as a foreign language that has the same difficulty for all learners.
For a first language, languages seem to have the same difficulty level. But that's very hard to test, for obvious reasons. All babies (barring cognitive or physical impairments) reach competency in their first language(s) at about the same time (4-5, roughly), but are not masters until many years later (if then). I hear native adult English speakers say horrible things like: I could have went (instead of: could have gone), without realizing it's a mistake.
For an English speaker, among natural languages, Portuguese is a level one language, per the American Foreign Service Institute. It requires about 600 hours of study to reach competency (not fluency). Along with most (but not all) other Germanic languages and all the other Romance (Latin) languages. There are far more similarities than differences between English and Portuguese, when you look at languages in higher difficulty levels.
For an English speaker, the hardest languages in the AFSI list are in level V, requiring 2200 hours of study: Japanese, Korean, the Chinese languages like Mandarin, and Arabic. With the exception of Arabic, those languages do not even have definite or indefinite articles, yet they are far more difficult, because almost everything is different.
However, a Japanese speaker would find Korean easier to learn than English (or Portuguese), despite having evolved from different languages (they are not related), because their grammars both rely heavily on agglutination followed closely by inflection. Languages with similar grammar types tend to have many other attributes in common.. They also both borrowed many words from Chinese languages, so those words will be familiar. And most East Asian languages, due to direct contact with each other, tend to have counters for numbers, no definite or indefinite or partitive articles, and either no plural/singular distinction or it's optional.
Similarly, a French, Spanish, Italian, Romanian, Romansh, or Catalan, etc speaker would find Portuguese far easier than any non-Romance language.
French definite articles: le, la, les.
French indefinite articles: un, une, de ( or d'). (de/d' - are negative indefinite articles, although there are times when un/une are used instead, even in the negative).
French partitive articles: de la, du, des, de l', de (d' -- negative again).
Italian definite articles: il, lo, i, glie, la, le.
Italian indefinite articles: uno, un, una, (and usually nothing in the negative).
Italian partitive articles: del, dello, dei, degli, della, delle, or nothing.
German definite articles: der, die, das, dem, den, des (each one has at least two different functions. Two of them have four. six words for 16 distinct uses).
German indefinite articles: ein, eine, eines, einem, einen, einer. - affirmative ones. kein, keine, keines, keinem, keinen, keiner -- negative ones.
(German has three grammatical genders in the singular; a different set of forms for the plural, where gender is lost; and four grammatical cases).
You cannot accurately judge a language based on one element. As you can see, there are languages with either equally hard or even harder articles than Portuguese.
And to emphasize, you cannot accurately say that Portuguese is among the hardest languages of the world, for everyone. The languages the learner already knows matter. Your belief that Portuguese is so hard comes from a desire for your language to be special. I see this with native English speakers as well. Some insist English is very hard, while others insist it is very easy -- for everybody in the world. Actual learners of English confirm the difficulty level depends on what they already know.
A language's difficulty depends on:
1. vocabulary (similar or not)
2. grammar (which can be similar even when the languages are not related)
3. pronunciation (there many factors in pronunciation: sound inventory, syllable structure, stress, tone, pitch, speed, contractions, insertions, syllable restructuring, etc).
4. culture. (the more different a culture it is, the more likely the language has concepts that are utterly alien to your own language).
note: Japanese is supposed to be among the hardest languages for an English speaker, but it was my fourth language. I found it less difficult than I would have if it had been my first language, with no knowledge of linguistics or phonology.