It's hard to say what the hardest constraint will be, at this point. Imaging and scanning are definitely hard obstacles; right now even computational power is a hard obstacle. There are 100 trillion synapses in the brain, none of which are simple. It's reasonable to assume you could need a KB (likely more tbh) to represent each one faithfully (for things like neurotransmitter binding rates on both ends, neurotransmitter concentrations, general morphology, secondary factors like reuptake), none of which is constant. That means 100 petabytes just to represent the brain. Then you have to simulate it, probably at submillisecond resolution. So you'd have 100 petabytes of actively changing values every millisecond or less. That's 100k petaflops, at a bare, bare, baaaare minimum, more like an exaflop.

This ignores neurons since there are only like 86 billion of them, but they could be sufficiently more complex than synapses that they'd actually be the dominant factor. Who knows.

This also ignores glia, since most people don't know anything about glia and most people assume that they don't do much with computation. Of course, when we have all the neurons represented perfectly, I'm sure we'll discover the glia need to be in there, too. There are about as many glia as neurons (3x more in the cortex, the part that makes you you, coloquially), and I've never seen any estimate of how many connections they have [1].

Bottom line: we almost certainly need exaflops to simulate a replicated brain, maybe zettaflops to be safe. Even with current exponential growth rates [2] (and assuming brain simulation can be simply parallelized (it can't)), that's like 45 years away. That sounds sorta soon, but I'm way more likely to be underestimating the scale of the problem than overestimating it, and that's how long until we can even begin trying. How long until we can meaningfully use those zettaflops is much, much longer.

[1] I finished my PhD two months ago and my knowledge of glia is already outdated. We were taught glia outnumbered neurons 10-to-1: apparently this is no longer thought to be the case. https://en.wikipedia.org/wiki/Glia#Total_number

[2] https://en.wikipedia.org/wiki/FLOPS#/media/File:Supercompute...