I personally like complex champions, the only problem with them is that Riot loves to add one or more mobility spells for them, so they become broken and either completly useless or dominating.
Ekko for example has a very complex kit, but luckily, it's not broken, he's a bit too strong right now, but nothing that cannot be fixed by simple number tweaking.
Why is Ekko healthy ? He has only one mobility spell (Even if its really annoying) and the rest of his spells needs proper targeting, positionning, and the enemy to postionnate wrong (Q, W and R). That gives him alot of counterplay. Riven on the other side is clamed broken, because too many mobility and powerful shield when AD, making counterplaying hard because we dont have any spell countering mobility well, and there is few you can do.
Same for LB, since she's point n' click, she care less about her or enemy's postionning and either burst hard, or fail to burst, making her pretty much binary and either useful of complete bs. Tha is a broken gameplay. Same goes for Zed, Katarina and most other assassins, since they are all point and click assassins (Zed's Q doesnt really count as a skillshot for me, when you ult its easy to land it)
Back to the topic, simple champions can also be toxic, Annie, Kayle and old Ryze where considered broken before, mostly because of their point and click abilites, Kayle was dominating, now you barely see her anymore cause of ehr nerf, Annie is dominating botlane right now, and Ryze got a rework.
And it depends what you maen with simple champion. A champion that is easy for begginers ? Point and click are easy for them because skillshots are harder to hit, but they are pretty much binary. A champion with a skill without any additionnal passive on it (Like Vel'Koz) ? They may be simple in form, but they can be hard to play (Vel'Koz isnt too easy to play), old Cassiopeia was considered pretty hard to play while her kit was fairly simple (Now she's fairly uninteressting).