The example of conjunctions shows one reason why you can't just draw up a list of all parts of speech in use in real languages. Because the more you look at languages the more complicated they get. And you're left with increasingly small sets of words that act in particular ways. To what extent can these sets be seen as subgroups of parts of speech, to what extent are they parts of speech themselves, to what extent are they all basically the same part of speech but with specific lexical constraints on certain specimens, to what extent are these irregularities in fact only semantic rather than grammatical.... or are irregularities due to certain words in fact underlyingly being two different words that sound the same?
For instance, "and" and "but". We can say:
1) You went to the shop and I went to the house
and
2) You went to the shop but I went to the house
So they look like the same part of speech. But then:
3) You and I went to the shop
*4) You but I went to the shop
Why are these different? Some possible reasons:
a) They are both conjunctions, 4 is impossible purely for semantic reasons because it makes no sense
b) They are both conjunctions, but 4 is impossible because 'but' has a specific rule that says it can only be used between two nouns if at least one is qualified or negated
c) They are both conjunctions, but 'and' is one subtype and 'but' is another. The first subtype can conjoin any strings of equal status, but the second cannot conjoin bare nominal phrases - however, adding a qualifier lifts a phrase above the status of bare nominal phrase, and hence 'but' can conjoin things with qualifiers
d) As above, but these are two different parts of speech
e) Both words are the same POS (which cannot conjoin bare nominal phrases) - but 'and' is homophonous with another word 'and' that can, and can only, conjoin bare nominal phrases (in some other languages, these 'two' words are not the same).
And then look more closely at 'but':
*5) X but Y
6) not X but Y
Hence what I said about qualifiers. And yet:
*7) some X but Y
So in e), we might want to say that 'but' is a clause-level conjunction, but that 'not... but' is a phrase-level conjunction. But then what about 'never... but sometimes' and so on? Are all these variants all lexical items, or are they one lexical item ('but') with complicated rules?
And then you've got 'therefore', 'while', 'yet' and so on, which are never phrase-level conjunctions, but do work as clause-level conjunctions. But these don't work all the same way either, because while:

I went to the shop while I was eating a pygmy
and
9) I went to the shop therefore I was eating a pygmy,
nonetheless
*10) I went to the shop therefore eating a pygmy
even though
11) I went to the shop while eating a pygmy
And yet probably
?12) I was going to the shop, therefore eating a pygmy
[OK, the semantics there make it iffy, but try "I was on the street after eight, therefore breaking curfew". Now, is this an issue of how 'therefore' can be used, or is it an issue of when repeated subjects and verbs can be elided?]
So which things do you call 'conjunctions'? All of them? Some of them? And how do these categories line up with the category you label 'conjunctions' in another language?