We try to take considerable care in kerning our Arabic fonts, including those we work on for other foundries. Obviously what needs attention are right-joining letters followed by isolated or initial letters within words. In some calligraphic styles, kerning across word-boundaries is also desirable where swash final forms dip under following words.

What I think of as the basic kerning of Arabic is fairly easy to do, but gets more complicated when text contains vowels or other marks: then kerning can result in situations in which either the mark positions need to be contextually adjusted (traditional technique based on scribal practice) or the kerning needs to be contextually adjusted, or both. The Adobe Arabic fonts that we made for Adobe use a combination of both mark and kern adjustments depending on context.

Sometimes the budget for a project is such, and the expecation of vocalised text so limited, that we can't afford to spend much time on contextual adjustments. Because most Arabic text does not use vowel marks, there are seriously diminishing returns to the large amount of work required to define all the contextual adjustments. Some clients balk at the cost.