Sure, a certain author's writing can be too difficult for some readers.
But that's why we have the concept of reading levels, which measures an individual's ability to handle complex writing.
But reading skill is kind of weird because the speed and ease with which we read is very much determined by our ability to recognize patterns--a good reader doesn't even have to look at every word in a sentence before their brain has picked out the important words and anticipated what is being said based on comprehension and convention. That's why, for example, we don't even notice most small typos.

So, if a writer's prose doesn't follow language conventions--if their sentence structure is weird or they go way too heavy on the euphemisms--then even a very skilled reader is going to have to grind to a halt and start unpacking each sentence piece-by piece, which is exactly what you don't want good readers to have to do.

Of course there is also variation in language convention. It's going to be easier to read a Regency-era novel if you're familiar with how English was spoken in the Regency Era... but those are still conventions, which a skilled reader can learn and adapt to.

Whereas "purple prose" (this term showed up in dictionaries in 1598, by the way) doesn't quite follow any particular linguistic population's language conventions--it's just that author ignoring the rules and writing whatever sounds good to them.

Clarity.
You can be as loquacious as you want if you arrange your words so that the structure, cadence, and meaning of each sentence is clear.
It only becomes "purple prose" when your ornamentation gets in the way of the meaning and flow of the sentence. If people have to slow down, back up, and read through several times to figure out how the hell that sentence was supposed to be read, that's purple.