The game has be updated to the latest version, at 2.2.0, which means there are some changes. One change is a Power Rally Boost, which rewards the player for completing the stage without fail consecutively by providing free items, which is another way to encourage players to make progress.
Perhaps the thing that was changed that affected players the most is that the assistants Shy Guy and Banzai Bill have reduced effectiveness. Previously they have a flat -10 SP decrease, which means that when using certain doctors it allowed for consecutive usage of their skill. The doctors affected by this are Goomba Tower (Level 3), Koopa Troopa (Level 3), Dolphin (Level 3) and Luigi (Level 5). This was abused in stage mode for easy clears and Daily Booster for maximum rewards, and it became more and more well known that this is the update that sealed the fate of this exploit. Basically, the -10SP bonus is changed to -10% SP, which is a huge nerf because using the doctors I mentioned, each assistant only reduced SP by 2.
Previously it was kept a secret for fear that making it known will cause it to be patched, since this is a free-to-play game with random character drawing, meaning that such an exploit alleviated some of the game's free-to-play shortcomings. I suppose since it's patched there's no reason to keep mum about it any longer. Did I take advantage of this exploit? You bet I did. That's the reason I managed to get most of my assistants and almost half of my doctors to Level 4, with even a handful getting max level. However, I was only able to take advantage of it after the developers put a limit to Daily Booster so I can't get unlimited coins/doc tokens, which is to say I wasn't able to get a maxed out roster.
Thank you for reading.