Still going through it (Playing a post-game file, cleared Xenologue 3 on Maddening recently), but to share a few stray thoughts...:
Nil is saved by the fact that he's a Dragon and thus can use certain Emblems really well even at Bond Level 1. Have him with Byleth currently, but I can see Corrin and Camilla giving him utility thanks to Dragon Vein. Veronica's also good since summoning doesn't care about his stats.
By virtue of being a Dragon, Nel too is usable. More usable thanks to having actual stats, but I don't see her having amazing combat outside of Engaging with Tiki.
Soren's the MVP of XL3. Not because he has Rexcalibur (I gave him to Gregory who can't get it without Bond Levels), but because he has Assign Decoy for messing with flier / cannon targeting.
Zelestia's very competent. Power of flying magic, yo. Boosted more by Chrom.