I think you got unlucky, I multi-box regularly and I have made over 100+ accounts that have completed tutorial island, and I've had success with power mining, motherlode (Upper Level is 10x easier), fishing, cooking, crafting, construction, runecrafting, agility (sucks when you fall), thieving (afkable methods), every bank standing skill, splashing, & blast furnace. While training combat stats is difficult, I've had runs doing sand crabs with no issues and others where I have (For F2P chickens & barbarians are the 2 best for multiboxing). Hunting, Farming, NMZ, and anything that requires a lot of clicks , involves RNG, and don't have consistent task starts to get difficult but not impossible. I've had success with simple quests like romeo, imp catcher, and goblin diplomacy. Non of the 100+ accounts have been banned for multi-boxing (I believe), I think the bans mainly came from botting instead (Small possibility that multiboxing was the reason but I doubt due to unique actions unless they manually check all the accounts and see that the clicks are repeated). But I'm not sure, just don't get reported and multi box with all accounts in different worlds, (Unless questing, Motherlode (Upper Level), and fishing).