If anything, I feel Rakan hit what would be the good middle between versatility and balance.
He has a 'lot' of tool on paper and is indeed the first real mobile support who can be very versatile but he got a ton of weakness in fight.
Rakan has no inherent tankyness other than a shield that is weak at early level. He is dependent of the defensive mastery tree to get more stat and a shield that allow his all in to be more safe.
Whilz he got a heal and a shield, his heal scale with lvl and ap, while asking for you to hit this low range skillshot AND hit a champion
If anythinf, his q is max last because of how ureliable and weak the scaling feel. It has good base dmg but it is mandatory if you think of the range and the fact ir only hit one target.
His W is very powerful and a major part of his power budget. It give him aoe hard cc along with mobility. But the hitbox is actually not gigantic and the delay make it actually fairly easy to dodgz. Rakan need to predict the movement or to follow up to make strong plah. A well balanced powerful spell.
The E is super good, the shield is good and it allow him to make very slippery play. Of course it will frustrate people to have a support who can get awqy from his mistake but what he gain on this part, he loose it in term of tankyness and dmg. Rakan doesnt trade like a nami (the other 'versatile' support).
His R is actually underestimated. It is like his kit, very versatile and disruptive. It ls an excellent engage, a very good peel, a good escape. You can engage on someone, see the ennemy jungler appear and turn to charm him while you w or e to escape.
What make him strong is his capscity to make the choice on his play.
he get fairly bullied in lane by poke comp, early bully (karma) and hard engage. BUT he can also work fairly well against all of them. He truly shine in organised play and I can see him be a good pick on lcs.
Xayah is more simple to describe.
Her massive stength is her minion wave control, which automatically make her good for lcs/pro play as she promotes rotation well. Her lack of escape is made up for strong dmg input and scaling, she snowball actually quite hard and is a good duelist.
she is good, solid but doesnt excel so much that other top tiers adc will be ignored for her.
she just has a massive waveclear control which also come with some good poke and easy trade. Even if her ult has such long cooldown and she isnt a mobile adc, adding she can also require proper feather set up to maximise her dmg input in duel (even with her W), I think she can pick in the lcs.
She is also fairly easy to balance, it is all about the number on her E is she scales too quickly and also lowering the dmg made by the feathers early on secondary target if her pushing power early is too much.
Overall, two very fitting and fun kit that dont feel overbearing to fight. They are fun to play as and against.
for me, it is a very very good release by Riot