There's no short answer. Well, there's no good short answer.
Trends come and go in cycling. Those trends include ideas on fit. Sometimes we want little tiny bikes, sometimes we want great big ones. Right now, little tiny bikes are the hot ticket and you have to bear that in mind: a lot of folks believe that small bikes are absolutely critical, because they've been told so. That's all.
Having said that, there are advantages to smaller bikes, along with disadvantages. The same is true with big bikes.
Small bikes are lighter and stiffer, on average. Light is always nice, of course. Prospective buyers should take note of any weight differences between sizes, however. They're often the equivalent of skipping the extra bagel. "Stiffer" is a mixed bag. A stiffer bottom bracket may or may not allow better power transfer. There's some intelligent argument from both sides of that coin. A stiffer front end handles better, almost without question. A stiffer frame overall usually results in a harsher ride, but IMO tire width and pressure is far more important when discussing ride quality.
Smaller bikes also tend to force a rider into a lower and more compact position. This may or may not be what you want. And yes, a small bike can usually be made to fit like a larger one with appropriate parts and adjustments. I'm not quite sure why someone looking for an upright position would want to start out with a small frame, but I see it quite a bit.
Larger frames tend to be a bit more flexible. Riders looking for comfort appreciate this. Riders looking for speed at all costs don't. Neither group is wrong. Larger frames allow you a more upright position, which is good for long distance riders and those looking for greater comfort. There's no easy way to make a big frame fit like a small one, so racer wannabes avoid them.
Steering is affected strongly by several factors, none of which have anything to do with size or stem length.
Assuming similar seat heights and bottom bracket heights, I don't see how frame size will affect ease of mounting, unless you are a complete idiot.
If you tend to hop off the bike with both feet on the ground when straddling the top tube, then you certainly want a low top tube. If you don't tend to hop off the bike with both feet on the ground while straddling the top tube (and it's certainly not necessary; I don't think I've done it in 30 years of cycling) then you don't need to worry about it.
So I guess the closest I can get to a short answer is "Size depends on your needs, and once you've figured out your needs, why would you buy a frame that's either too big or too small?"