Backing up Lightning channels is difficult due to the complexity compared to backing up on-chain funds. Two problems arise when backing up of Lightning funds. First, the backup needs to be updated every time the state of the channel changes (which happens several times during a single payment). Second: if you restore channels from a backup that you think is up to date, but is actually an outdated stale state, then your counterparty could believe you are attempting to cheat on them and take all your money as a punishment.