I have heard about them going to implement SegWit and I don't know if that's done yet or not. I also know they used to have problems on the estimation of the fees etc.
It's done. They finally added Segwit.
About the fees, I haven't used Mycelium for a while, but I'm pretty sure they allow you to customize your fees starting with 1 sat.
I know about Samourai as well, and I don't like the UI to be honest and I also read somewhere that they made a bold move by basically remove the fiat price of BTC in the app which leaves the user clueless and could lead to mistakes.
They did. Their argument is that 1 BTC = 1 BTC. Honestly, I don't think that's such a big deal if you're looking for a good wallet with many functionalities. You can always look at the price with other apps.