10% is not a "big advantage" over the PC. If it was a big advantage, I wouldn't suggest it. I guess this is preference, but achievements that don't do anything are meaningless to me. Why would I kill 47 summoned units with cavalry under a full moon if I get nothing for it? As a primarily single player game, there's not going to be much use for a social aspect to them, unlike say SC2's portraits.
Depending on what it's 10% of, it is. 10% gold income is a really big advantage, that's 10% more money to support a larger army. Having a larger army the start of every encounter because of bonus gold the computer doesn't get is a pretty huge advantage, really. It's an advantage over myself from the last game where I didn't have it.
People do achievements because they want to, not because they're needed. What you're describing is more like the deeds system from Lord of the Rings Online, and that quickly turned into nothing more then a massive grind fest in order to get them all because in the harder areas of the game, those bonuses matter.
This will be the same thing if it does something meaningful. Want to jack up the game to really hard difficulties? You better go grind out all your achievement bonuses first. It becomes a time sink and not some fun extra optional challenge.
Even though ME is a different game, it's also single player and that's why they're comparable. The only consideration is the one player, so any balance concerns pretty much go out the window, as long as the bonus given isn't overpowering (which something like 5-10% is not, or +1 to stats, and the like). There's a lot of flexibility with a rewarding achievement system. For example, you might be able to choose 1 or 2 bonuses to take into a game. So if you feel like playing an RPG-ish game with just your Sovereign, you can pick two unlocks that benefit him. If you feel like playing an economic game next, you can pick those instead. If you for some reason feel like it makes things too easy, you don't have to pick anything. Because it is a single player sandbox, this system makes perfect sense as it allows each player to customize their game experience to their tastes. Plus it seems like a good compromise since you wouldn't have to use the bonuses if you're really against them. It would also make it fine for "competitive" MP (in quotes because it's never gonna happen ), or just disable it altogether.
The system makes no sense in a sandbox. The whole point of a sandbox game is that starting it is hitting the global reset button from the last game. Carrying bonuses in doesn't do that at all.
