It is hard if not impossible to keep a game balanced with so much content being added to it in such a small period of time.
Not to mention that the skill level of the League designers and developers is of ex-Warcraft 3 modding level which lead to much spaghetti code and little in-depth sight on the actual game creation.
In the past years they developed from that to a really serious business so there is not much to look down to any more, but whatever.
As I said, it is extremely hard to keep League balanced.
Imagine changing one item? How many people use that item on which champions? Do these champions decrease/increase their power level by the change of this item? etc etc etc.
Bringing a new champion to the rift also has to be balanced with hundreds of items, hundreds of champions and thousands of matchups.
I have seen games where I play as an underdog where the game is so terribly unbalanced that the opposing team cannot do shit against us. Why? You do not know. How can you change that? You do not know. So balancing is hard to do and imho it is very well done.
Also, it is kind of the meta of the games.
Last time I spoke about this tanks in general were buffed (the juggernaut meta). Response? Noone played assassins because they would be useless anyway. So assassins got buffed. Response? Noone played squishy champions any more so they got some nice protection items. Response? You can actually go on forever and predict upcoming meta
But the big point is that game changes are also done based on how many people play a specific champion.
Specific champions being OP is not always as it seems it is.
I recall Irelia who was insanely overpowered and people were continually complaining about her "Better nerf Irelia!".
Then on one day a patch came out and in the patch notes, you could see Irelia got nerfed. So everyone was happy.
The patch after that, they said that the changes were made 6 patches earlier and just not added to the patch notes.
Hence the meme "Better nerf Irelia!".
I believe skill level is much more important than meta. And as long as you enjoy playing your champions, you should go for it.
I play Maokai APC Mid for 2 years now. (Stupid koreans are now copying me.) And main Ziggs even longer than I am level 30.
I like to play those champions, so I play them, independent from what I am against.