At least the rank 4 of Liferoot Staff (heals 6 per nature spell) works fine. Lot of equipment have bugs at lower ranks where they do absolutely nothing, so I wouldn’t be very surprised if this is another one of those. That said, of course it’s good to also remember that some enemies can block all healing (including Mi’da boss fight).
What comes to nature comp for PvE: it’s a strong team, but you cannot beat everything with just one team, no matter how good the team is. So be prepared to diversify and build a new team when you hit a roadblock. You can also check Mercenaries Bounty guide for some teams proven to work. Finally, in before the “you cannot expect people to drop thousands of dollars to buy every merc in game to build all those teams”… I have every merc, they’re all at max skills & gear, and I’ve spent zero dimes on the game. Of course it takes time to get them, but it’s not impossible at all.