Different languages have different sets of phonemes. For instance, in Japanese, "L" and "R" are allophones, meaning native speakers consider the sounds to be the same. So, take these two UBI's:
http://ubicate.me/NALADU-MAMOKE
http://ubicate.me/NARADU-MAMOKE
Smack dab in the middle of Tokyo, about 15 minutes apart from each other.
Also, this would require speakers having the same vowel distribution/phonological rules. I pronounce "PACASU-FADERO" and "PACASU-FEDERO" pretty similarly. Relying on letters that are only different in voicing is also risky. For instance, take "P" and "B". It's easy to mistake "PACASU-BATERO" for "PACASU-PATERO".
Finally, street names have an advantage over UBI's. Street names are usually formed from words in the inhabitants' native language, so they sound more natural and will of course be easier to pronounce.