Is it? Certainly, multiple sequences of characters map to the same sound: the final vowels in et, est, thé, années, soleil, clef all sound identical (at least to my ear: there may be some subtleties of pronounciaton that escape me).
But from what little study I've done of French, I don't remember the opposite problem being as bad: a given sequence of characters is generally pronounced in one way. So if you come across an unfamiliar French word, you can take a punt at how it's pronounced, and the odds are you'll be correct.
There are of course exceptions, especially with proper nouns (Moët, as in the champagne, is pronounced moe-ETTE not moe-AY, Freddie Mercury not withstanding), but these seem to be fewer than in English.
[Edited for grammar.]
While I think that indeed a given sequence of characters is generally pronounced the same what is confusing for children is the plethora of consonants that are not pronounced. So ils chantent and il ment. écoles et les and so on... There's also the liaisons that adds a lot of confusion between what's written and what's actually pronounced. But maybe you're right and this is more of a problem in English.
I think this is a rule of the trema - the vowel is not diphtongued and the following consonant is not silent. Words like Noël, Anaïs are pronounced the same way (no-ELLE and ana-YSSE).