I have to admit that I never actually studied it formally, but when I did look at it, it seemed pretty straightforward. The Latin sounds are preserved well in many words (such as cruce, for example).

As for Spanish vowels being "pure", I don't see how one can say that. The mutation of o > ue and e > ie is overwhelming, so where the Italian costare yields the inflected form costa, the Spanish costar yields cuesta. There are some verb forms where Italian has the same e > ie shift as Spanish (tiene, viene), but in non-verbal settings it retains the long e of Latin (i.e., bene, venerdì, merda, cento, festa, inverno, greco, etc.).

The mutation of consonants, of course, is far more advanced in Spanish, with c > g, t > d, f > h, pl > ll and the pronunciation of c and z with a decided lisp in Castilian, as well as the shift of v > b in sound (if not in writing), which you noted earlier.

Not only that, but there are certain bizarrities in Spanish, like how Latin perna, which meant the upper leg of a pig, became the word for a human leg. Also, why did the Latin equus disappear, replaced by cavallus (Sp. caballo) but equa remained for a female horse (Sp. yegua)?

There are Arabic loanwords, like albañil for "mason", and Basque loanwords like zorro.

While Italian does have some odd Arabic loanwords here and there, like ragazzo (from ragaṣ, messenger) and arsenale (from dar as-sina'a, workshop), they are far more infrequent.

And that's without even getting to all the American Indian words that litter Latin American Spanish like so many armadillo roadkills on a Texas highway.