It was a clever Sif who saw the reality of what Loki had done. He changed one rule, halfway through an arranged plan, and the entire construct, the whole exercise completely fell apart. She watched Loki laugh maniacally, just as happy to destroy the construct as he was to own it.
This was the lesson Sif was thinking of when Thor first told her of Loki's plan to build a team of Nortlander Raiders to spread and strengthen their masks in the southlands. That was a reasonable and believable reason to commit to mortal heroes, to be sure. But what was the second part of the plan? Why did he need this team of mythic heroes?