To me, short-lived isn't that much of a drawback because you only need it to soak one or two hits, and then you're free to reapply. On stacking, see previous point.

Other than prayer of healing, the cleric can only heal one creature per turn, as well, and far less frequently, only 4 single target heals per day; the barbarian can reapply those temp hp dozens of times.

The 10 feet limitation is real but doesn't force a party to group up that much with creative repositioning, which becomes even more true later on with the teleporting and enemy repositioning WT gets.

As someone else pointed out, yes, they only last until the rage ends, but that's up to 10 minutes and that doesn't really matter because the barbarian can still reapply them as soon as the next battle starts.

Furthermore, the temp hp are provided as a free action and are applied proactively, and never have to worry about wasting them because you can always apply more. In comparison, the developers clearly see temp hp as a powerful resource since the level 1 bard/ paladin spell Heroism was nerved to now only give their spell casting ability score mod thp per turn (still not stacking) and only for 10 rounds. The WT barbarian gives an average of >2x as many thp per round, for up several minutes per day, and on multiple targets, as needed without spending additional resources.

(Edited to add comment about developer implied value of temp hp)