> Any idea if this is why, in Japanese-dubbed anime, the voice actors seriously mangle some English words/names? E.g., they often add a vowel sound to the ends of English words that should end with a percussive syllable.

I don't know anything about anime, and little about Japanese, but I think Japanese (and Chinese) have a fairly strict consonant-vowel form for all their syllables. That makes foreign words that have runs of consonants or do not end it a vowel hard to pronounce, so speakers of those languages have a tendency to insert extra vowels to make pronunciation easier for themselves.

It's kind of like how English speakers will usually change the Pinyin "X" (as in Xi Jinping) into an English S or SH sound when they try to speak it, because the actual sound doesn't exist in English.

Actually, it's probably a simpler reason than that. The Japanese language is largely a CV syllable string (consisting of a consonant and vowels); consonant clusters do not exist, and the only final consonant permitted is 'n'. English, by contrast, is a much more phonotactically complex language--consonants can pretty freely appear both before and after vowels in a syllable, and English also has several consonant clusters. Imagine trying to pronounce the word "strengths" if your native language lacks consonant clusters--it's like an English person trying to pronounce the Czech phrase "Strč prst skrz krk". On top of that, Japan is not great at English proficiency (it's definitely weaker than any other rich country, see https://www.ef.com/wwen/epi/).

It's not really that the written language makes the names hard for them to pronounce, it's that the spoken language doesn't make it easy, and there's probably not enough care to try to pronounce them. Where the written language does make it hard, it's usually when people try to localize Japanese media into foreign languages, and the intended references in names are lost because of the mangling process of transcription into katakana.

As an English speaker who has traveled to Japan without learning much of the Japanese language, I agree generally but I also noticed that there are some cases where a vowel is written but not pronounced. For example, "gosaimasu" is mostly pronounced without the "u" (creating a counterpoint against final consonant other than "n" being forbidden) and "gozaimashita" is mostly pronounced without the second "i" (creating a counterpoint against consonant clusters such as "sht" being forbidden). It gives me the impression that these rules exist more in written Japanese than spoken Japanese, at which point it becomes less clear why adding a vowel to the end of foreign/imported words is so common. Maybe it's just my English perception that the sounds /s/ and /sh/ consist of pronouncing only a consonant, when in reality the fact that those sounds have duration (not just a moment) actually means it's more of a vowel even when totally unvoiced!

As I think on this further, even these voiceless /s/ and /sh/ sounds involve putting the lips into either an /u/ or an /i/ shape based on the following vowel even if that is also voiceless, creating that which is not a syllable in English, but perhaps is for this purpose in Japanese. The C-V cadence and final vowel (given lack of final -n) rules are satisfied...

There's a finite and relatively small number of possible syllables in several Asian languages, including Japanese.

I think it's more that Japanese speakers just don't have those types of sounds in their phonetic repertoire. Some may be able to pronounce them, but most will not (and may not even notice the difference).

Every person has a certain limited set of consonants, vowels, diphtongs, triphtongs, tones, and even syllables that they are able to recognize and reproduce. This is something you can train to recognize more, but you will probably never be able to pronounce or even distinguish the totality of all those used in all languages, even just the living languages on Earth.

Even if you did, there is an added complication that some languages actually used multiple sounds interchangeably, and explicitly distinguishing them may actually confuse you. For example, most European languages recognize various consonants as the same "R" sound, even though they are vastly different (French R is a back of the throat trill, Italian R is a trill near the palate, and English R is articulated next to the palate without any trill). If you come from a language where these are distinct sounds, you may have trouble understanding that two people who use different R sounds are pronouncing the same word.

There is also the R/L problem, A sound that to me, a native english speaker, is fairly distinct. However these are the same sound in Japanese. Because of this I think that it is very hard for Japanese speakers to figure out which one to use and they get switched all the time.

First, that is not because of writing system specifically, but because of the rhythmic structure of Japanese language, see, for example, this video:

https://www.youtube.com/watch?v=J_HLY0Rss-g

Second, in Japanese dubs these words are not usually actual English words, but Japanese words originated as borrowings from English language, so voice actors don't actually mangle them, the same way as English speaking people don't mangle the word "coffee" as they usually pronounce it, despite it being different from how Italians pronounce "caffè".