**Fixed typo**
The difficulty lies in design, imo. Achieving different tiers, etc. is a matter of time and resource acquisition. A bit of luck and a lot of waiting/mining and you pretty much have all paths open to you.
The real challenge in sandbox games is design. Building something from scratch, when you know that you start in a place that is not significantly representative of the end state, mean that a large portion of good design requires a lot of forward planning. Space, energy, resource, and time considerations are all apart of the process.
The focus of any person's FTB progression should be cross-mod integration. Since the value of a modpack, is in fact having multiple mods to play with. It has taken me 3-4 worlds to fully develop my system of energy gen/transfer, item/resource management/acquisition, tool set acquisition/maintenance, and building strategies (resource and time management).
Understanding key features of all mods, and how they work in conjunction with one another require one to know the finer specifics of each individual mod. IC2 back in 1.4.7 with GT installed was a volatile place to be for a noob. Something as simple as plugging a batbox to a line of LV tin cable meant losing all that cable. Antics of EU/t vs. EU/p and how to properly transform EU both ways required a lot of playing around.
You also had to consider that various machines only took MJ, such that focusing on building a system which centralizes on EU production, meant strategies for handling MJ were required. Yes, you could build multiple, separate energy systems, but from an engineering standpoint you are being quite inefficient with your resources. You will never be able to fully utilize the full potential of your system.
Choices of which end-game tier of armour/tools/weapons, or even machine you wanted to use was a matter of what type of tech path/mod-favoured systems you have built. Nano/quantum require a relatively robust EU generation system, storing ~ 100 million EU in total. Even at 512 EU/t generation that's 200 000 ticks, or 10 000 seconds (3 hours +).
Where does RoC stand? It sounds to me like RoC is a tier based, all-inclusive, do everything tech related mod. This is similar to IC2 and TE. I feel that (after watching the fusion reactor tutorial by Reika) that RoC/ReC(?) attempts to utilize multi-blocks as much as possible, avoiding the 1-block solution type of deal. This is the same as IC2 GT. Additionally, requiring one to understand that a moment = force and how fast you choose to use that moment (rotational speed) will make you think about what you are doing. Similarly to IC2 EU, which is a pain to deal with and one mistake meant blowing up some pretty expensive multiblocks.
The reason why TE is popular, is because there are so many other things to do in FTB. Not everybody playing this game is an engineer, scientist, or mathematician. Already, I can observe the psychology of our scientist players, laymen players, and personal to myself, engineer players. What each of us want and get from FTB is very different.
So, in the end if you want to know why RoC exists, is because Factorization exists beside IC2, TE, BC, etc. All have their own power system, item/liquid/energy transport, and machines that (whether in a system or singular) do precisely the same things. This is what people want from modded Minecraft and is what sets it apart from vanilla. This is why almost all the major mods focus in on these key things. Even TC4 has evolved into a realm which a new genre of automation is being developed.