As a person who owns this game and played it, I do agree it does have problems, but they never bothered me.
Basically, the story is Dr. Eggman has created a new scheme to take over the world, and after the first stage, Sonic is defeated by the mad scientist, especially since he has the aid of Infinite, a new villain for this game. After Sonic's defeat and imprisonment in space, Eggman goes on to take over 99% of the planet, but Knuckles forms a resistance, alongside some of Sonic's friends, to fight back. The story sounds a lot like Sonic SatAM, but getting rid of characters like Sally Acorn and replacing them with the game cast. However, they get a new member, the Rookie, which is a character you get to create using one of seven different species, each having a special ability, and being able to use Wispons, weapons using wisps' color powers without needing the wielder to transform. Once you make your Avatar (as the game will call it), he or she is sent on a mission to break Sonic out, eventually leading to the two teaming up, Also, Classic Sonic joins the fight, coming to the Modern Sonic dimension due the events of Sonic Mania.
So, the game allows you to create your own character for use in Avatar stages as well as the stages where he/she teams up with Sonic. However, one of the problems with it is how limited the selections are. You are limited to one Avatar until you beat the story, and customization options are limited from the start, as new clothing and Wispons are unlocked as you progress through the story as well as complete specific tasks. As for customization, don't expect to make a perfect replica of characters like Sally Acorn or any other non-game character, because the customization has it where shirt and pants are one single outfit, and females are forced to wear bodysuits (although you can make it the same color as the skin, but it will still show).
Depending on the stage, you play as Sonic, Classic Sonic, or the Avatar. Some stages have Sonic and the Avatar teaming up. Each one controls differently (the Avatars have a wire attack and can use Wispons, Sonic has his usual homing attack and can boost using white wisps, and Classic Sonic plays like he did in Sonic Generations). Sadly, another problem is the game can be beaten very quickly if you decide to go straight to the goal, with it being extended only if one decides to go for the collectibles like Red Star Rings that are hidden in almost every level. Also, some of the challenges can be a bit grindy, with some ranging from killing a certain number of enemies with a certain Wispon, to getting a certain medal rank as a certain species. While completionists and achievement hunters will find this nice, others will find it excessive and boring.
However, to ease the grinding, the game adds daily challenges that will add a score multiplier when completed. Sadly, these are insulting easy, as some will require you to change a certain article of clothing on your Avatar.
Other gripes include Episode Shadow, which acts like a prequel, and when beaten, unlocks Shadow for play in Modern Sonic's stages (the episode only has three stages, and Shadow is nothing more than a reskin of Sonic) and the fact that Classic Sonic was put in this game to link it to Sonic Mania (and solidify the game's entry as an anniversary title). Otherwise, the game is okay and is worth checking out if you are a Sonic fan.
