Using Flutter has been extremely helpful for us in a startup environment, getting apps built for both iOS and Android in half the time (or less) compared to doing both. Our team has plenty of native mobile experience and has built separate native apps that mirror functionality in the past.

Flutter was so much more efficient, it actually lives up to the promise of write-once, build to multiple platforms.

As for design, we have a very custom design and in Flutter you are in no way locked into Material design.