A lot of what you'll find called "phonics" in mainstream bookshops is of variable quality. In among the chaff there is some wheat, but you have to be very clear about what you're looking for.
Most high-quality synthetic phonics materials are at present only available from specialist suppliers or online (see Phonics Resources).
The "sounds of letters" rather than the spellings of sounds
A lot of "early phonics" materials teach the "sounds of letters" along "e is for elephant, u is for umbrella, x is for xylophone" lines.
This suggests to learners that the only interesting letter in a word is the first one, and encourages them to read by looking at the first letter and guessing (these words are so long and hard, what else can they do?).
It also suggests to them that there are only 26 sounds in English, when in fact there are 441.
Mainstream bookshops usually have some materials that teach basic one-letter-one-sound relationships fairly well in short words like "cat", "men", "him", "on" and "up".
However they often don't include the regular ending spellings ck, ff, ll, ss, ll, ve, tch and dge, and some don't cover the consonant sounds without their own letters – sh, ch, th and ng (English also has a zh sound, as in beige and vision, but that's used mainly with harder vowel spellings).
Too many sounds too quickly
A lot of promising-looking materials too-quickly mix in words with consonant blends like "flat", "pegs", "slip", "pond" and "grub", or even triple-consonants like the "spr" in "spring" and the "nks" in "tanks".
For many learners with weak phonemic awareness or other language difficulties, that's too many sounds all at once. They need to consolidate one consonant either side of the vowel first.