Japanese pronunciation is quite simple. A word is one or more syllables and a syllable is one of the things represented by hiragana. Learn that, you've got most of the sounds. The exceptions are devoicing of 'i' and 'u' in some contexts, and standalone 'n' changing to 'm' in some cases (pronounced shimbun rather than shinbun as it is spelled). That's about it for that part.

Then, there's pitch accent. You can get this wrong and it almost never matters (yay, context!), but you will sound foreign/weird. Then again, pitch accent is not the same in all dialects so there's that as well.

Japanese sentence order is SOV (I school go) instead of English (SVO). I assume you're talking about being easy to learn as an English speaker since you wrote this post in English.

If we go further down the rabbithole, there are different registers that use different honorifics, verbs, conjugations, and such. It's annoying (personal opinion), but you just need to remember 'say x instead of y in this context'.

Mandarin has more possible syllables and sounds and also has tones. I imagine messing up the tone is also saved by context usually, but it seems more severe than pitch accent mistakes. It seems grammatically more similar to English from what I can tell and lacks verb conjugation. I've only taken one or two lessons, so that's about all I can say.