I see the inverse of this, with native speakers of languages that use ABC (alphabet) to learn a language that’s logographic such as Japanese & Mandarin (since both have 漢字) as a single character is a word (difficult concept for them to grasp at first). However, even if let’s say a Mandarin speaker with zero exposure to English leans ABC for the first time, would the grammar be hard even if they’ve managed to master the alphabet?
There are sounds that don’t exist in both JP & ZH like words that start or end with “TH” (i.e. Thorough, Moth) as they don’t have that in their phonology. Don’t even mention the misleading pronunciations of certain words in ENG based on spelling that stump them like “Colonel” (大佐・上校) since most speakers of ENG say that as “Kernel”. Even if “alphabet” based languages are easy, the grammar can be hard for a non-native to learn.
As in, what would it be like for a Japanese & Mandarin speaker learning a Euro language (all alphabetical based) from 漢字 > ABC or Cyrillic as it’s alien to their own plus huge differences in vocab with zero similarity (like this: “Salida” from ES is equivalent to 出口 in JP & ZH) including common things most Euro languages have: gender cases (no such thing on JP & ZH), definite and indefinite articles for example to list a few.
Grammar is probably the wrong word, but it seems like the question is about word identification, which relates to how spoken and written language interacts.
For instance, a local dialect may choose a different set of sounds for a word but maintain the same logogram as other dialects, so there is greater mutual intelligibly across the written language over the spoken language. In contrast, an alphabet written language encodes the sounds into the written text, so the written word looks different across different dialects reflecting the spoken language.