Money market fund vs. cash sweep vs. external bank for uninvested cash?

Now that rates are much higher in paying more attention to my uninvested assets in my retirement and brokerage accounts. Typically these are held in sweep accounts, but moving to a MM fund or external savings can yield 4%+. On my retirement accounts obviously an external savings is off the table, but what are the disadvantages to a MM fund other than the fact that to switch gears and invest the assets is gonna be a 2-day delay vs. a sweep account? Under normal low rate circumstance this might be an issue, but markets don’t typically swing that much in two days so the delay might be a worthwhile trade off. Transferring from an external account would be a similar delay. What am I missing? MM funds can’t typically go negative in any circumstances from what I’m seeing.