Linguistics

By "poorly designed" I'm conveying "full of sub-optimal decisions that introduce unnecessary complexity and unintended consequences". Justin B. Rye has a full rant on that; I'd like to pick specifically the following issues:

• excessively large consonant set, full of uncommon distinctions like /w/ vs. /v/, /x/ vs. /h/, /ts/ vs. /tʃ/
• almost no concern for phonotactics
• over-reliance on vowel alternations to convey morphological distinctions
• case marking and articles at the same time, requiring you to learn two systems when one could do

In special, Esperanto as defined in the 16 rules is full of assumptions on how a language works that boil down to "you should know it, because it works like in European sprachbund languages". And sometimes those assumptions break even for those European languages.

Later auxiliary constructed languages show a lot of improvements in this regard. And while they do focus often on one or another aspect, as you hinted, often the result is cleaner.

That it uses internationalisms

The source of the internationalisms is often a disputed point on itself. It relies for example a lot on Romance and Latin vocab, even when it doesn't make much sense (e.g. "sango" comes to my mind).