The short answer is, the formerly used Romanian Cyrillic alphabet (used before 1830 when the transition towards Latin started and many reforms ensued) is more suitable for the phonology of the Romanian language than the current Romanian Latin alphabet.
The long answer will first establish the definitions of alphabets and phonemics, then proceed to define the criteria for suitability, and then make a case for each alphabet by examination.
This question aims to compare the relative efficiency with which the two scripts (graphology) that have codified the Romanian language (at one time or another) depict the sounds (phonology) of the Romanian language.
The graphology or graphemics of a language is the study of the writing system(s) of a language, as represented by concrete written symbols or glyphs inventoried as the graphemes of said language. Glyphs represent all the different equivalent representations with which one can render the same symbol, or grapheme. For example, one can use different fonts to generate different glyphs a, ɑ of the same grapheme, which can be written as <a>, since using either a or ɑ doesn’t change the meaning of a word.
A grapheme constitutes the smallest unit of a writing system of a language and it may or may not correspond to a single phoneme of that language. Graphemesadmit allographs. Allographs are any glyphs that are considered a variant of a grapheme.
The phonology or phonemics of a language is the study of the speech sounds or phones of that language, many of which (but not all) are inventoried as the phonemes of said language.
A phoneme is a basic distinct and meaningful unit of speech sound as recognized by the speakers of that language, bearing subjectivity in that phonemes are relative to each language. For example the phoneme /r/ exists in both English and Romanian, but it represents slightly different sounds which also have different usage distributions in each language. Phonemes admit a secondary level of distinction defined by variations in the realization of the sound, which are called allophones. For example, in Romanian the phoneme /h/ admits three different allophones:
- [h] in its most frequent usages, such as before all vowels except “i”, such as in harpă (“harp”); it’s the same sound as in the English “harp”;
- [x] in word-final positions such as ceh (“Czech”) and before consonants, such as in hrișcă (“buckwheat”) – it sounds like the Scottish pronunciation of the word “loch”;
- [ç] before [i], such as in histrionic (“histrionic”) and [j], such as in the popular pronunciation of hienă (hyena), pronounced [‘çjenə], when the normative pronunciation is [çi’enə]; it sounds like the English “h” in “human”.
The subjectivity in the case of “h” lies in that, although it is pronounced in slightly different ways, the native Romanian speakers identify it as the same sound, therefore the same phoneme. As a result, it is rendered with the same grapheme as well.
So graphology or graphemics arranges glyphs by subsuming them under graphemes, which admit allographs, while phonology or phonemics arranges phones by subsuming them under phonemes which admit allophones.
The Cyrillic as well as the Latin alphabets are adapted to the languages which have used them at one time or another, and since Romanian was written in Cyrillic from some time in the Middle Ages until the mid XIXth century when it was completely replaced by the Latin script, there was an adapted Romanian Cyrillic alphabet the same as there currently is a Romanian Latin alphabet. If you want to learn more about the history of these two alphabets in Romanian, you can see my previous post on Why Did Romanians Switch to the Latin Alphabet?
Since they are both segmental scripts where graphemes correspond to phonemes (they both trace their origin back to the Greek alphabet), then the main criterion ought to be which script contains the more complete set of graphemes which uniquely corresponds to all or most Romanian phonemes.
For that let us list all of the Romanian phonemes with their allophones, and their corresponding graphemes and allographs in Cyrillic and Latin. You can see in the table below I have assigned a score to each discrepancy, whether there are multiple allographs to a grapheme denoting a phoneme, whether there are multiple graphemes denoting the same phoneme or certain phonemes without their own unique grapheme.
As it can be seen, both alphabets score very closely, with Cyrillic having a slightly better standing. The criteria of my examination has been followed very tightly with the expectation that most important allophones of the language’s phonemes ought to have their own unique grapheme. A language satisfying this condition is improbable to be found among the Indo-European languages utilizing the Latin alphabet, and the Romanian language is already recognized as being one of the most phonetic from that group.
That being said, for all of the other, non-phonemic reasons I have listed in my previous answer to Why Did Romanians Switch to the Latin Alphabet?, the Latin alphabet is more suitable for the Romanian language overall.