I don't know how likely it is for big baddies to get through the tesseract itself, but the bigger or more challenging types definitely have a better chance of getting past the armed guards. I can think of plenty from my world that could come through and take out the entire welcoming committee without even blinking an eye, and I know my world's not the only one with beasts like that. But if that happens, at least people like me will be here to take them on, just like we did/will do at home.