According to https://www.howmanysyllables.com/, “dune” and “rider” are 1 syllable and 2 syllables respectively, but “Dunerider” is 4 syllables.
I don't understand.
The first word of a close compound noun is usually stressed, so “Dunerider” should pronounce [ˈd(j)uːnraɪdə], right?
I feel "Dunerider" should be ˈd(j)uːn-raɪ-də, 3 syllables. If it is 4 syllables, is it ˈd(j)uː-n-raɪ-də?
related question: A better parody of “traitors, Rattlesnakes and alligators”
The website you linked is unreliable and incorrect. I wouldn't recommend using it (or any other website) syllabification because syllabification is a highly controversial topic in linguistics. How to syllabify a word depends on who pronounces it. For example, the traditional pronunciation of the word film is [fɪlm] (one syllable), but I've certainly heard the disyllabic pronunciation [ˈfɪ.ləm].
The word 'dune' is usually pronounced [djuːn] and 'rider' [ˈɹaɪdə] (BrE). I agree with your analysis that 'dunerider' should be [ˈdjuːn.ɹaɪ.də] (three syllables) (it can also be pronounced in many other ways, though, even four-syllable pronunciation is possible depending on the speaker). It is not pronounced /ˈd(j)uː-n-raɪ-də/ (four syllables), because it suggests that the /n/ is syllabic which it is not. /n/ can only be syllabic when it's preceded by an obstruent (/t s z d/ etc) as in button. I would suggest you use your ears for deciding the number of syllables in a word.
Let's see how 'dunerider' is a three-syllable word:
This is the sonority curve for the word 'dunerider'. The dark blue dots mark the peaks of sonority and the number of peaks correspond to the number of syllables. As you can see, there are three peaks in [ˈdjuːn.ɹaɪ.də], hence three syllables.
The sonority of all phonemes of English can be depicted on a sonority scale (sonority hierarchy). A sonority hierarchy is a hierarchical ranking of speech sounds. Typical order of sonority values is:
Vowels [ɑ, ɔ, ɪ, i] etc > Glides [j, w] > Liquids [ɹ, l] > Nasals [m, n, ŋ] > Fricatives [s, f, θ, ð, z, ʃ] etc > Affricates [d͡ʒ, t͡ʃ]> Plosives [p, b, t, d, k, g]
Vowels are the most sonorous whereas plosives are the least sonorous sounds.
[From my previous answer]
I have explained the Onset Maximisation Principle (syllabification rule) in this answer.
There is a "Contact Us" form linked directly from the page you cited with the text "Wondering why dunerider is 4 syllables? Contact Us! We'll explain." If you want to know why the site has it listed as 4 syllables, try that.
I would question using this syllable website as a source of truth. It would be better to understand for yourself the rules to determine the number of syllables in a word. To this end, please see this question, you may find it helpful.
As for dunerider, your analysis is correct. If it is said as a combination of dune ("doon" - 1 syllable) and rider ("rai-der" - 2 syllables), then it is 3 syllables. That is how I would naturally say this as a native American English speaker without a pronunciation guide.
The web site is completely, totally wrong.
Every single word I tried - was wrong.
It's not a serious, professional site. (It's not offered by, say, a major dictionary or the like.)
It's a silly site someone threw together, the "data" there means utterly nothing.
There are dozens of toy "syllable counter!" website sitting dead on the web, SEO companies throw these up when there's a fad. This QA should be deleted promptly as it is only feeding a (ridiculous, dead anyway) spam site.
