Adding a backness distinction between low vowels, æ vs ɑ, as in Farsi, and/or adding front rounded vowels y, ø, Finnish-style, does not disrupt the equilibrium. Nor does adding back unrounded vowels ɯ, ʌ, Korean-style.
Nor does a tense-lax distinction, pairing i with ɪ and u with ʊ as in English or German.
From this typological perspective English is however rather messy — particularly the types of English where historical monophthongs have developed into assorted diphthongs. (Yes, it’s GOAT that I’m thinking of particularly.) The Anglo-American NURSE vowel, ɜ or ɝ, doesn’t actually disrupt the balance, but from an international perspective is an extremely unusual type of strong vowel.
The prize for unbalanced vowel systems seems to go to Pacaás Novos, aka Wari’, an Amazonian language spoken near the Bolivia/Brazil border. It has four mid/high front vowels, but only one back vowel. More unusual still in Wari’ is one of the items to be found in the consonant system, namely the ‘dental affricate’ t͡ʙ̥, which consists of a dental stop plus a bilabial trill, together forming a single unit.
Dan Everett, co-author of a descriptive grammar of the language, says that the Wari’
are a helpful, kind people who speak a groovy language.
(The above photo of a Wari’ man relaxing was taken by the anthropologist Beth Conklin.)
Sadly for us, though, younger speakers are reportedly simplifying their consonant system. Rather than articulate t͡ʙ̥ they prefer to use just the first part of the affricate, so reducing it to t.