Using exersaucer/bumbo/etc?

sap05e

Is anyone's LO using something like this yet? DD has good head control now, but I've heard both my mom and aunt say stuff like baby's spines or back/neck muscles aren't strong enough yet. My LO is 11 weeks old today and has been trying to sit up from a laying position for a few weeks - like baby ab crunches, lol. She also keeps her legs extended and stiff when I pick her up now and likes it when I let her "stand". I'd like to get her out of seats that contribute to flat head as soon as it's safe. I'm just unsure when that is..

Last weekend my uncle sat her on the edge of a table and let her feet dangle while she sat up (while hands still around her of course) and my aunt had a cow about her not being strong enough for that yet. I didn't say anything but I do it all the time. Is that something that just older generations believe or is it a real concern?