I really can't see how you read anything else that "it's very optimized" in my message, tbh.
This reads to me as it being a PITA to maintain. Cross platform code is usually a pain, cross platform with assembly optimizations is more of a pain. Optimization nearly always makes things harder to maintain, and this sounds like it was optimized to hell and back...
Not more than any other language: when it's well done, it's manageable. When it's spaghetti code, it's not manageable.