Individual xp is problematic. Clearly this is my opinion and not shared by all.
If a player misses and adventure, we make up some in game story for why they aren't there, and their character still receives xp. If a new player joins the party, they get xp matching the current players. If players participate in the adventure, they get the same xp as anyone else.
If a player is missing a ridiculous amount of sessions and it's disruptive to the game, that's a separate issue and needs to be dealt with outside the game. Same for if someone is phoning it in with effort to play the game. There are ways to reward RP without tossing around XP.
Good role players are good, bad ones are bad. You'll seldom turn a half way decent player into a great one while bribing them with xp. However you will make people bitter when someone is getting rewarded for something and other people aren't. XP rewards are subjective, and unless you are infallible (just to be clear, you're not), chances are you will make a mistake, or players will disagree with your decisions. Most people will just go along with it and not complain because they don't rock the boat. That doesn't mean they agree or are content with your ruling, just that it's not worth their effort to risk a friendship over something as minor as a few xp.
I've seen good players rewarded fairly time and time again only to have their character start to get noticeably more powerful than others in the group because of it. I've seen GMs that favor a player, or a certain type of role playing and reward it/them more creating imbalances. I've seen people fall behind in xp due to an occasional absence or reward xp and then they start to resent the GM or other players. I've seen those players quit, often citing other excuses but after discussions finding out that it was due to the XP. I've seen players get upset because the adventure doesn't give them an area to apply their expertise or role play their character in a meaningful way while another character has had the limelight for a couple adventures.
I've seen cowardly GMs intentionally do xp related gimicks specifically to drive players away by constantly giving them the short-end of the stick. I've seen bonus rewards of XP lead to ridiculous situations where people are trying way to hard to get the bonus xp that it becomes disruptive. My favorite story of this was a player in a D&D campaign where the GM rewarded xp for role playing or for using class related skills. After several sessions of mages, warriors, and rogues getting loads of bonus xp while the bard barely had anything to do, the bard player went off the deep end in order to get xp. He literally started play music and singing EVERYWHERE. We go to the bar, music, we go to the weapon shop, music, we take the road out of town to the castle on the hill, music, we talk to the guard captain, music. It became annoying, but was also funny as the guy had a valid complaint about what was happening and found a funny way to exploit it.
This is one of those areas where the potential pros simply don't outweigh the potential cons. More games will be ruined, more people will be hurt, more people will silently stew with anger, then will be encouraged to play the game when dealing with xp like this.
Remember folks. It's a game. Games are meant to be fun. Punishing people isn't fun. Creating inequality or inequity isn't fun.