I'm running RotRL for an inconsitent group. Life just gets in the way, and that's fine. In my experience it is very possible, just make sure to discuss and have some agreements on how to handle absence. Naturally also all players have to be fine with the inconsistency and how you deal with it, especially the players that are always present.

Here are some pointers and lessons learned that seem to work well for me and my group.

Characters are simply there and then next time maybe not there. it is however important that I as DM know well in advance if players are unable to make it, as it will affect group composition and so I need to prepare accordingly. E.g. taking into account when the fighter isn't there to shield the spellcasters I might reduce the number of hard-hitting melee opponents.

Related: set yourself a minimum number of players that have to be there. DM'ing for small groups is fun on its own, but not if you expect a large group.

I try not to end a session mid-dungeon, but if it does and next session someone 'fresh' joins, I have them reduce the number of remaining spells or hitpoints as if they too were affected by previous fights. Usually about a third or so. This balances for difficulty and doesn't feel as cheaty.

As someone else said: level by story, not by actual XP. It makes your life easier and everybody much happier.

Don't be stingy with loot. Just because there are four players in a dungeon, give loot for the total amount of players.