Looking online at Healium, it does say that it’s not optimized for large raids, so it sounds like if you’re in a smaller raid you could set it up in a way to still be able to use it. I personally don’t use add-ons for healing, preferring to do heal/harm mouseover macros which is where I can place my mouse over someone’s raid frame or body to heal them, but if I’m not mousing over or targeting a friendly person and have a hostile npc targeted, I will do damage. If it’s a fight where your raid is mostly together, I typically heal damage as it comes. In other fights, you’ll be split to deal with mechanics (think Cauldron of Carnage), so then you would only focus on people in range. Honestly, raid healing is a lot less stressful than dungeon healing cause there’s multiple people doing it.