Power alone isn't a guarantee that you will win. It's just an indicator of how much you've upgraded your toon/ship - gear, level, abilities, etc. Synergy is important (and overall powercreep, Home One Rebels are certainly nowhere near as good as Profundity). You can throw in your strongest ships, if they don't work together, you're more than likely to lose. As for Sith, I'm assuming you mean Leviathan. These last few months in particular have been messy. They changed things around so that TIE Dagger is necessary for the opening line-up, but it was completely RNG dependent for a while. Thankfully, they fixed it so that even if you go second, you can still win by calling in Scimitar as your first reinforcement. It doesn't matter if it survives or is destroyed at any point, Scimitar's reinforcement ability (gives all Sith allies extra speed) is still active.