/rgh/ isn’t the only consonant cluster “allowed”.  /w'/ and /y'/ occur as well.  Remember the /’/ qaghwI’ (a.k.a. aleph or the glottal stop) is also a consonant in Klingon – as it is in Arabic for example -- and should be properly enunciated.  The following pairs of words sound different to Klingons:

maw offend maw' be crazy

paw arrive paw' collide

bey howl,  yell bey' ceremonial display

Duy agent Duy' defect

As for why these clusters and not others, well that’s one of those “just because” rules we have to accept.

--
Voragh

From: mayqel qunenoS

Is it known, why the {rgh} is the only consonant cluster which is allowed at the end of a word ?

What makes it so special with regards to any other consonant clusters ?

~ nIghma'

On Oct 31, 2017 16:34, "SuStel" <sustel@trimboli.name> wrote:

On 10/31/2017 4:16 AM, mayqel qunenoS wrote:

Neither does the SIla'.

~ nIghma'

On Oct 31, 2017 10:09 AM, "mayqel qunenoS" <mihkoun@gmail.com> wrote:

Neither 'uSu' nor ghevI' follow the cvc klingon phonology.

~ nIghma'

The complete Klingon phonology (C = consonant; V = vowel):

CV
CVC
CVw'
CVy'
CVrgh

excluding:

Cow
Cow'
Cuw
Cuw'

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'uSu' = 'u + Su' = CV + CVC
ghevI' = ghe + vI' = CV + CVC
SIla' = SI + la' = CV + CVC