I guess I could see some usefulness in Sound Families, but I don't think they're really all that needed. Tags work really well in conjunction with global variables, and calling each individual sound isn't that big of a deal, especially since you can call sounds by name now rather than have to use a drag and drop list, in case you wanted to call sounds via variables or something. Still, it might streamline certain soundsets a bit more depending upon your project, though I doubt I'd ever have need for Sound Families (and I'm a musician!).
Are you talking about intro tracks transitioning seamlessly into the loop section, or the loop section itself not repeating seamlessly?
For the first one, are you experiencing this while previewing or are did you play it uploaded online? I've experienced an issue during previewing where the intro and loop sections had a noticeable gap, but when they were uploaded to dropbox and played online the gap was gone. Of course, I had to make sure the loop was preloaded at the start of the layout before the intro finished so that it could just play it instantly. I also opted to import the music as "Sounds" and play them as "Sounds", mostly because I feel I get a better sound and better loop.
If you're talking about the loop having a gap between repetitions, then I don't know what to tell you. I tend to test on Chrome and all my looping tracks loop fine (at least when they're played online).