Technically he’s not really strong but 380 healing over 5 second and every 10 mana 3 second moonfire that hits enemy heroes during the 5 seconds of healing gives ALL heroes with regrowth an additional 180 healing immediately for each hero hit hit 2 heroes and every friendly with regrowth has 360 of immediate healing
Add Wild Growth you extend regrowth by 1 second per enemy hit. so regrowth last longer so healing from dmg is available more, Natures balance or Harmony at 13 and you can see
His D, gives mana, and increases cooldowns for the hero its given.
Pretty much read the tooltips and it becomes apparent how he can be good.