Well, I'm large (and tall) and that sort of thing looks dreadful on me - the 'ship in full sail' effect. Love tunics, but much prefer something that defines my hour glass figure more. But haven't seen these particular jumpers.
The only problem I have had with tops that have that sort of handkerchief hem, is making sure that the shortest bit is still long enough to generously cover my tummy.
The long bits at the side look fine, but then the hem curves up towards my belly and somehow the curve seems to accentuate my belly, possibly because a normal top may look slightly shorter at the front as it goes over the tummy so a top that actually is shorter at the front fools the observer into thinking the tummy is huge.
I don't avoid tunics which allow definition of my waist, either through their cut or because they look good with a belt. These look good on me with jeans or trousers that fit my long, slimmish legs closely.
But I do avoid those of sort OP mentioned, for the reasons above.
not flattering, ok if you are tall and have big long legs jutting out from under it, but otherwise would make you look much larger IMO. I'm a short 14 and would look an 18 in that! I need to wear stuff that shows that I do go in A LITTLE under my boobs before going back out again (even if its not in as much as it should be!)