When judging whether a fill is acceptable, I guess there are two concerns. One is the overall difficulty, which I don't find that hard to judge. While even the most erudite polyglot has lacunae, it evens out.
But then there are those weakest links. Let us say your fill is beautiful, just the right mix of everyday words and interesting oddities - but one word stands out as possibly unacceptable. Where do you draw the line and reject the entire fill because of this one word? How do you know what is simply too obscure?
One area that I find difficult to judge is botany and biology. There are a million and one genuses that are of interest to only the most focused expert. But then there are those that are in everyday use. I think that 'amanita' is reasonably well known as a class of poisonous mushrooms. And I would think 'sirenia' (sea cows) rings a bell for at least some solvers. But then I wanted to use 'weigela', a bush that gets plenty of google hits on gardening sites. I have never heard of it personally, but then I'm not into gardening. How do I know if this is something that enough solvers will recognize, or something which will just make them mad? Mind you, we're not talking about overall difficulty level here, but is it obscure enough that I should throw away an otherwise excellent fill? How do I know?
Another area is variant spellings. As a rule, I just don't accept them unless I can find they are actually in common use. It seems many dictionaries list variants that have not been used for centuries, if ever. I know as a solver, it makes me mad when a word is used that must reasonably be called a misspelling. I'm using a Scrabble word list for preliminary judgement, because it contains a ton of words with definitions. But it is terrible in this regard. A 'tercel' is apparently the British variant of 'tiercel', a male falcon. My instinct tells me 'tiercel' is correct, but then again this too is obscure enough that I can't be entirely sure. And then there is 'tarcel', apparently a male hawk. But I just can't justify using a word where every top Google hit is a Scrabble or crossword site. It will have to go, at any cost.