It took me a while to trace this down, but I was getting incorrect results for some of transmitted values on a mac, namely nothing higher than 0x3F would come through (while the results on windows were correct), I found a list of exceptions in serial.cs, Please consider changing (this is from line 228)
case RuntimePlatform.WindowsEditor:
case RuntimePlatform.WindowsPlayer:
case RuntimePlatform.WindowsWebPlayer:
case RuntimePlatform.WindowsEditor:
case RuntimePlatform.WindowsPlayer:
case RuntimePlatform.WindowsWebPlayer:
case RuntimePlatform.OSXEditor:
case RuntimePlatform.OSXPlayer: