Not a problem with diacritics. But I wish languages used them consistently, so that e.g. o/ö or e/ę relate in the same or at least in a similar way in all languages that use them.
For phonemic spelling systems, it wouldn't even be hard. Start with the baseline of aeiou for the basic vowel triangle. Then let's say that umlaut changes the vowel's "default" backness without changing roundness, ogonek lowers the vowel, tilde makes it nasal, and breve makes it a glide. This gives us aąäãăeęëẽĕiįïĩĭoǫöõŏuųüũŭ, plus all combinations of these diacritics when needed (which should be very rare). Add doubling to indicate vowel length, and I can't think of any European language that cannot be consistently expressed this way; American English should only need aąäeëiįouų.
Yet Spanish is a very expressive language, and it has no trouble expressing whatever you want using fewer vowels (note there are more than just five vowel sounds even in Spanish, it's just that variations don't have different meaning).
There's a lot of sound variation in regional dialects of Spanish, from country to country and even regions within a country. People pronounce vowels and consonants differently, and even the "accent" varies a lot (some people can seem as if they were "singing" instead of speaking, to speakers of other countries/regions).
what's cool is that regional dialects have different vowel/consonant pronunciation but the differences are almost completely consistent internally and between the dialects. Like, Peruvians pronounce anything with "y" or "ll" like an English "j" and Argentines like "sh", always. Everything maps 1 to 1.
Spanish isn't particularly rich in sounds but other languages use combination of vowels or accents. French or Portuguese have much more than 5 sounds and the same vowels.
Portuguese has the same basic characters except for ç, in the case of vowels it's resolved with combinations of letters or by adding accents. For instance e and é have different sounds and ou isn't the same sound as o followed by u.
