fast charge doesn’t actually mean anything.
it’s a marketing word.
it is safe to charge at a batteries maximum amperage rating. in other words a 20 amp discharge rated battery can also charge at 20 amp.
charge the same battery at 40 amps and it will overheat go into thermal runaway and catch fire.
unless your talking about hot charging. and if you are then no it isn’t safe. you are bringing the pack to the edge of runaway and holding it there for 30 minutes to and hour because your have to have everything right now.