"He develops serenity and insight coupled together" is originally found in the Pali sutta MN 149, as follows:
When the noble eightfold path is developed, the following are fully
developed: the four kinds of mindfulness meditation, the four right
efforts, the four bases of... power, the five faculties, the five
powers and the seven awakening factors.
Tassa evaṃ imaṃ ariyaṃ aṭṭhaṅgikaṃ maggaṃ bhāvayato cattāropi
satipaṭṭhānā bhāvanāpāripūriṃ gacchanti, cattāropi sammappadhānā
bhāvanāpāripūriṃ gacchanti, cattāropi iddhipādā bhāvanāpāripūriṃ
gacchanti, pañcapi indriyāni bhāvanāpāripūriṃ gacchanti, pañcapi
balāni bhāvanāpāripūriṃ gacchanti, sattapi bojjhaṅgā bhāvanāpāripūriṃ
And these two qualities proceed in conjunction: Tassime dve dhammā
serenity and discernment. samatho ca vipassanā ca.
A reason why samatha can be unbalanced and not have vipassana is because the practitioner is actually not practising the Noble Path but, instead, is practising the Hindu style yogic manipulations that are actually taught in the Visuddimagga.
In the suttas (MN 118 and SN 48.10), the Buddha taught meditative factors are mature when they have the quality of "letting go" ("vossagga"). This is the practise of the Noble Path, namely, the "letting go" or "abandoning of craving".
If you try to watch the breathing, for example, or watch kasina coloured discs, and similar non-sense taught in the Visuddhimagga & Abhidhamma, this is craving; this is not practising the Path. This is why most 'Buddhists' make little progress with meditation. They try to cling to a meditation object, like a baby clings to its mother. They are too scared &/or too attached to wrong teachings to 'let go'.
As for vipassana (insight) being overbalanced, this sounds like illogical (even though there is one or two suttas that teach similar non-sense). Since the suttas (such as SN 22.59; MN 118; etc) correctly explain vipassana results in dispassion; then obviously a dispassionate mind will be calm or have samatha.