Didn't like the Bumbo, she always leaned over to the side to get out.
When we wanted her to practice sitting to strengthen her muscles, I put the Boppy pillow on the floor, propped up with another small pillow under it. So it kinda circled around her. It was only a month or so later that she was sitting up on her own, so I'd say just hold off and use the exersaucer when LO can sit up