Question

why decantation is considered unsuitable for separation of 2 immiscible liquids from a suspension

Answer 1

Decantation is generally used to separate liquid from heavy solid particles which settle down at the bottom the container.
Two immisicle liquids form an emulsion. They form different layers when allowed to stand for some time. The lighter liquid forms upper layer while the heavier liquid forms lower layer. They can be separated by using separating funnel. The heavier liquid can be drained out first from the bottom of funnel followed by lighter liquid in a separate beaker.