I'm honestly confused. What's the difference between Optimized and Safest?
Frankly, I don't even know what's the difference between High and Low Control.
In the universal scale, 'optimized' is not nearly as interested in you as you are in yourself (or others, even).
An optimal solution has to be impartial to as many external wants as possible.
Peace between all nations, for example, would be viewed as optimal. It would also mean many advancements in many areas. But it would also mean that many advancements/changes would not occur - socially, culturally, economically, technologically. It would still be the best, but with one issue - while peace solves issues between nations, it may not solve issues beyond that.
For a Mass Effect example - the incorporation of organic matter into a synthetic frame was the ideal-so-far solution to the Reapers. However, it never, in itself, could have ever addressed the greater problem of organic-synthetic *relations* in a good enough way.
Curing the Genophage with Wrex and Eve is great. Woo. Go for it. Cultural Renaissance! However, we don't know what goes on beyond that. Just as the krogan can rise into a more peaceful state - we know it has threatened the galaxy in some of the worst ways. This also requires dropping grudges against the krogan.
Peace is a dream. We can fight to achieve our dreams though.
~~~
Safety between most nations is different. Instead of going on a (relative) hope and prayer that things will be good if we just put things in exactly the right place, Safety is more about taking a wider approach to try to cover as much as possible, even at the cost of losing the most optimal path, at least for now. So alliances are made in the world, sure, but nations also don't open themselves up for exploitations of their open arms. This is co-operation with some others, not assimilation of everything else.
The downsides really are two things (compared to Optimal):
1)Well, you're not getting your solution now. You'll get it later in some form.
2)Others will still suffer, while those you protect will more definitely be okay.
People argue between the 3 paths in ME3 and for good reason, but the devs intended this to be a moral choice, for better or worse. All choices succeed, as long as you got the EMS. All have some sacrifice, but not 'too much'. This way, we can make our stand in terms of morality, just as ME2 had us make our stand in terms of method (Radiation to keep any of the tech as a weapon, or Explosion to destroy all of tech for its dangers).
Because ME3 ended more morally (instead of ME2's mechanically - how do you want to go about saving the galaxy?, or ME1's emotionally - do you care about the Council or Alliance?), Bioware outright knew it was going to have some problems with its reception, but there you go.
But yeah, Safety/Freedom/Peace are big themes, and they conflict with each other at times. Sometimes to be more Safe with what you can imagine, you gotta hold back on the Peace that might achieve the unimaginable. Not to mention the Freedom to achieve more with what you know.
Safety is an illusion. It is an illusion that you can control though.
~~~
There isn't a lot of difference between High and Low Control. That's part of the 'safety' aspect. Whatever downsides you get from it, they are somewhat recognizable, if kinda alien, and may be dealt with (as opposed to the downsides of Synthesis being utterly unknown and would have to blindside the galaxy).
Control ensures that everyone save Shepard, or more than otherwise, makes it through the war (that lasted until the Crucible). Earth might be wrecked in Low, sure, but the Normandy always makes it. It was a Control wave, not a Destructive one, so we can assume that it wasn't as outright powerful.
Low and High dialogue with the Catalyst is different partially because more possibilities opening is a bigger thing that it seems. While it means you can do more, it also means more potential uncertainty and debate, more questioning of the self, OR more solidifying of the self against the external.