just like acoustic waveguides with speakers, they really arent enough all on their own. they are good for keeping the passive components in the XO to a minimum, but they cannot replace the XO entirely. for starters if the tweeter is sent the full signal including bass that is out of its operating range, you will kill the tweeter. there is nothing wrong with using them as a tool, but they simply cannot do the full job in a multidriver IEM, they can be used to filter the output frequencies, but they cannot filter the distortion and or damage caused by sending them audio and power outside of their SOA
One thing to note is that unlike real tweeter, BA 'tweeters' don't get hurt from being fed with low frequency signal. To put it more precisely, there exists no real tweeters in BA world, only those that are better at producing high frequencies compared to others, which are later on called 'tweeter'. They all are full range drivers in the first place.
Anyway, you are correct about needing more than just some simple acoustics filters in a multi-casing iem (not multi-driver).