Not many have talk about it but what if lootbox become forcibly into laws, right now ESRB will not accept lootbox as gambling thus leading government to force it into law. This is a problem with all content rankings not such ESRB, if these content rankings do not do their job it leads to complex rules and laws. This is why lootbox is a mess in gaming, while it is ok in the USA not so much in China or Japan that have rules and if your game falls into it you are in risk of not only losing customers but money too.
I do not think you should only use the ESRB, it is a flaw system from the very start since any kid can buy AO games online or M rated games on steam store.
If you really want to do this right you got to have the IARC which protects game developers. This system can give warnings like games having nazi symbols, violence, noise that may not be ok to someone who went to war, realistic like killing, etc. ESRB does not have those things because like i said it is a fail system.