The only thing in power save mode it doesn't allow the screen to use120hz.For me, no. I work at a desk full time so I have my charger. In my car, I have fast charging so when I connect for android auto, it's charging there. I haven't had a situation where I'm without a charger for long periods.
Also, I keep all my phones in power save mode, irregardless of battery size.
I have an s10 and the battery is only slightly larger than the flip 3, 3500mAh. Did everyone complain about the battery life for the s10? I'm no power user. Screen time is usually less than 2 hours so my s10 is still between 40 and 50%. I don't really see that the flips battery will be that big of a problem.