Answer:
It will have a negative effect on the quality of your work for sure.
Without being able to properly and easily read the text, you will not be able to distinguish between symbols that are similar... like "," vs "." or ';' vs ":" or "m" vs "n" or "I" vs "i", and so on.
You might not be able to see where a sentence stops and you'll keep reading, mixing up sentences and wonder why this is in the same sentence.
If you're dealing with a language with accents (like French or Spanish for example), you will not be able to distinguish the accents ("ê" vs "ë" for example).