Playstyle is often decided on teamcomp. If you roll up an assassin that thrives in chaotic, hectic teamfights and your team is full of four squishy, longer ranged poking guys with no form of initiation... then you're going to have one boring game. Or you're just going to snap, go in, get murdered because you built full damage, and then yell at your team for never fighting.
Well of course they're never fighting! Your team has no front line, no one to peel, and no way to initiate a teamfight. Their strength is in poking the enemy team down, and that's what they should be doing. Sometimes rerolls aren't just for getting rid of Eve or a free week champ, sometimes they're there for you to try and get something your team lacks. If you rerolled your assassin into a tank with cc he can peel those squishies with, you'll have a more active roll in the game and contribute more to the fight overall.
Another thing I see too often is a lack of tanks, or tanky champions. A lot of players will roll up a Malphite in a team of four squishies, look who they got, and reroll for that Lux. Now your team may lack tanky stats and an initiate, and you may have just sabotaged yourself depending on who you rolled. Sometimes you might have to bite the bullet and play a champ you don't like because that's what your team needs to not get their teeth kicked in for twenty minutes. No one has fun in a game they pretty much can't win, that's why 4v5s are so horrible. Don't hamstring yourself by rerolling champions with traits your team could really use.
Finally, keep in mind that there is no jungle. That means there are no jungle items. That means there is no sated devourer. That means that a lot of champs that are strong on SR because of that item aren't necessarily strong on the HA.