Of course, it could have been worse. We could have ended up with French as the lingua franca (yes, I know what franca means) where there is almost no correlation between written and spoken language.
Going from spelling to pronunciation in French follows (admittedly complex) rules that are rarely broken except for common words (or endings such as -ent). Vowel pronunciations for a given spelling are far more variable in English, and often depend on the etymology of the word. Plus, English has word-level stress that is not marked in writing (French has none, and it's marked in Spanish), and moving the stress will usually make a word unintelligible! That alone makes writing => pronunciation very difficult.
Unsurprisingly, we can vaguely quantify this by looking at dyslexia amongst languages. English and various Southeast Asian languages that rely on Chinese ideographs are by far the worst, followed by things like Arabic, French, Hebrew, and German that have fewer exceptions but less guidance, and then followed last by things like Spanish, Cherokee, and so on that are truly one-to-one.
There are a number of ways currently used, but I have a new one to propose: compare the size of two G2P models (1 for each language), which have similar RMS errors. Assuming they are generated using similar techniques, the one which requires the bigger model probably has a less clean phoneme-to-grapheme correspondence.
Even ignoring all of these, its clearly not bijective. For example:
C --> /k/, /θ/
Z --> /θ/ [0]
K --> /k/
Q --> /k/
G --> /ɡ/, /x/
J --> /x/
N --> /n/ (with several distinct secondary articulations), /m/ (rarely)
M --> /m/
R --> Can be tapped or trilled.
Etc. You can go here and see many bijection-failures here: [1]
I am being intentionally unfair to Spanish (which truly does have a much, much better phoneme-grapheme correspondence than English[2]), mostly to illustrate the point that there aren't really any languages which have a 1:1 mapping between spellings and pronunciations. Even if you decide to use the IPA to write your language, non-standard dialects end up needing to read words that don't match their pronunciations. What happens when inevitably the language undergoes change - do we update all of the books to use the 'new' spellings of words?
The ideal orthography shouldn't be completely 1:1, but it should be relatively shallow. From that perspective, Spanish orthography is a fairly attractive option.
[0] The non-1:1 situation with /θ/ gets much worse in most dialects of Spanish, where it is not distinguished from /s/. See: https://en.wikipedia.org/wiki/Phonological_history_of_Spanis...
[1] https://en.wikipedia.org/wiki/Spanish_orthography#Alphabet_i...
[2] Look at how effective Spanish-speakers are at reading without "decoding" compared with Portuguese, which also has a good p-g correspondence. In particular, look how much faster the Spanish students are at pseudowords, on page 141: https://www.academia.edu/17872463/Differences_in_reading_acq...
The function from written Spanish to spoken Spanish (provided we are talking about a single dialect) is surjective, but darn close to bijective, especially if we exclude words of recent foreign origin.
> Many people expect … to predict the spelling from the pronunciations-- not realizing that few orthographies meet this goal. It's far from true of Spanish, for instance, which is often held up as an example of a good orthography. I stopped fervently admiring Spanish orthography when I saw a sign in a Mexican bakery with about one spelling mistake every third word.
So, no, hardly bijective!