The specific card in question requires you to defeat three Maniacs to gain an Exhibit item. What if you are already at the Monster Limit... then NO monsters appear (can you get the Exhibit Item for free?). Or (more likely), you can only place 1 or 2 Manaics (because of the Monster Limit)... so it becomes "easier" to get the Exhibit Item. Because of the nature of the "reward" for defeating the monsters, I was assuming that the monsters were a limited duration kind of thing.
Good to point out that these cards were from Curse of the Dark Pharaoh. I will repeat my question there (just in case).
I believe both of these cards are from Curse of the Dark Pharaoh (original version): this question probably should go there.
I think this is an instance where it's just trying to place a monster into that street space, not to immediately cause a monster combat on someone who is there.
Why would you make that assumption about the Mythos card? It's just like any Mythos card from the base game: some place monsters into the streets. They're just monsters on the board, like normal. They count against the limit. You only encounter monsters on the board during the Movement phase, as normal.