I was able to pull bosses out of the ground in the "All bosses" level, which I assume was intentional. What's not intentional is that I was able to let go of each boss on top of a wall to defeat them instantly. I didn't even have to fight at all!
The battle with two Bowsers is also easy depending on how you do it. If you walk slowly after collecting the Fire Flowers, only the SMB1 Bowser will appear. You can stop to fight it, and then after that, the SMB3 Bowser can be lured into the lava pit just before the bridge.
I did get an "out of memory" message after two of the fights, and the "Save Peach" room is just a black screen with no floor, but that might just be my computer.