Firstly, it is important to note that every person is going to be a bit different. One native english speaker might find Japanese to be the hardest thing ever while the other will struggle mightily with Arabic or vice versa. Personally I agree that Arabic is more difficult than Chinese which is logical and straightforward despite the fact that most non language enthusiasts just assume that Chinese is the hardest language in the world.
To just say that Arabic is an alphabet of 28 letters and that’s it is a real simplification. It is much more complicated than that and learning to read it well is much more difficult than you are making it out to be..Because short vowels are omitted you have to learn all kinds of rules and patterns for when a letter makes a certain sound and it is really technical and confusing. letters have 3 (or 4) different forms and works nothing like the alphabets we have in the west. Reading it is tied closely to the languages grammar which is freaking hard. It is technically much more challenging than Chinese
Japanese is also much more difficult to read than Chinese in my opinion because there are so many homophones and multiple kanji meanings.