For native speakers of most modern Romance languages (apart from some reminiscence of the neuter case), Nordic (Germanic) languages, English, Dutch, or Bulgarian, it is usually considered “difficult” to learn an inflected language like Latin, German or Russian: cases are a priori felt as too strange, too “archaic”, too ‘foreign’ to the own system of expressing ideas. However, for a common German, Baltic, Slavic, Greek speaker, or for non-IE speakers of Basque or Uralic languages (Finnish, Hungarian, Estonian), cases are the only way to express common concepts and ideas, and it was also the common way of expression for speakers of older versions of those very uninflected languages, like Old English, Old Norse or Classical Latin; and their speakers didn’t consider their languages “difficult” …
Therefore, to use different cases is the normal way to express concepts that non-inflected languages express in different ways – i.e. not “more easily”, but “differently”. That’s the point Esperantism has lost in its struggle to convince the world of its “easiness”. In fact, the idea that cases are difficult is so impregnated in Esperantism, that some did create “an old version” [probably deemed “more difficult”] of Esperanto called Arcaicam Esperantom, as a fiction of evolution from an older language…
Thus, among the European population (more than 700 million inhabitants), just around 200 million speak non-inflected languages, while the rest use at least 4 cases to express every possible concept. Within the current EU, more or less half of its speakers speak an inflected language – like German, Polish, Czech, Greek, Lithuanian, Slovenian, or non-IE Hungarian, Finnish, etc. – as their mother tongue.
For example, the literal sentence “I go to-the-house” [not exactly the common expression “I go home” which is expressed differently in each language] would be said in Spanish “voy a-la-casa”, or in French “je vais a-la-maison”, in Italian “vado a-la-casa”, etc. Therefore, in an “easy conlang” for Western European speakers, say in something called Esperanto, a sentence like “io vo a-lo-haus” is apparently “easy”, because the syntactical structure is similar to those non-inflected languages.
NOTE: In fact, there are other interesting concepts behind the use of the obligatory subject before the verb in languages like English or Esperanto, that appears usually in those languages that have reduced the verbal system; therefore, the subject is necessary only in those languages whose verbal inflection becomes too simple to express an idea that must still be expressed some way – more or less like different combinations of prepositions and articles are often needed to substitute the lost nominal inflection, as we discuss here. In those ‘less innovative’ languages that retain a rich verbal system, the subject appears for some reason, as e.g. in Spanish “yo voy a la casa”, which must be expressed differently in innovative languages, using different linguistic resources, like e.g. Eng. “I myself go to the house” (or maybe “it’s me who…“), or French “moi, je vais a la maison”. Is that obligatory subject and ‘simplified’ verbal system of Esperanto “easier”, and therefore “better”…? I guess not. It’s just an imitation of French or English that Mr. Zamenhoff deemed “better” for his creation to succeed, given the relevance of those languages (and its speakers’ acceptance) back in 1900…
On the other hand, in German it would be “Ich gehe nach-Haus-e”, in Latin, it is “vado ad-domu-m”; in Polish “idę do-dom-u” etc. The use of declensions, if compared to uninflected languages, is usually made of just a simple change of “preposition+article” -> “declension” – or, in the ‘worst’ case (as it is shown here), by a “preposition+article” -> “preposition+declension”.
To sum up, can some languages be considered “more difficult” than others? Yes, indeed. If seen from a European point of view, some linguistic features are not easy to learn: the Arab writing system, Chinese unending kanjis, Sino-Tibetan or Vietnamese tones, etc. can cause headaches to [adult] speakers willing to learn them… Also, from an English, French or Spanish point of view, learning a language like Esperanto might seem “better” because of its apparent and equivocal “easiness”… But, between (a) all Indo-European speakers learning a non-inflected language like English [or ‘easy’ Esperanto], or (b) all Indo-European speakers learning an inflected one like Proto-Indo-European?; I guess there is no language “easier” than other, and therefore the “better” option should come from other rational considerations, not just faith in the absurd ramblings of an illuminated Polish ophthalmologist.
Therefore, the question remains still the same: why on earth should any European willing to speak a common language select an invented one (from the thousand “super easy” ones available) than a natural one, like the ancestor of most of their mother tongues, Proto-Indo-European?