There is clearly an effect - but marginal. Dynamic with 60Hz is already running in eco mode, so not good as a comparison mode. So don't expect to gain much, unless you compare with 120Hz Performance Mode. However, it's definitely not bugged, if by that you mean not working as advertised.
From what I've seen battery saver usually adds an hour+ sot, especially on a device with a large sot number to begin with.
The device is silky smooth always with a powerful cpu, i expected the saver modes to slow down its usage a bit since I can't manually do it without rooting