They're rarely stronger, usually they get buffed because performance on them (as people learn them) tends to be poor, then as people get better it gets obvious whether buffs were needed or not in the first place and they get reassessed. This process can take up to a couple of months. Then it may depends on WHERE a champ sees success. Have people found an ideal build? Are they still playing it in a role it wasn't originally intended for?
A new champ has the advantage of enemies not knowing how to play around you yet, while you have the disadvantage of not knowing the champ. They're not purposefully shipped to be overpowered, but Riot is trying to push their boundaries every release. Making something new that isn't quite like the other champs before, maybe they want to introduce an entirely new mechanic and aren't sure where to gauge it power wise etc. since every champ has a certain "power budget" allocated to their abilities and base stats. A champ with a really strong ultimate (Neeko) has a really lackluster passive (use decreases based on vision and enemy awareness in general as they can tell you're just faking), because they took some of the power from that and put it elsewhere. If you know this, you can see the same system applied to every champ. Often this is shown in the base stats or mana costs, sometimes its long cooldowns on crucial non-ultimate abilities if not used properly.