The problem with that is that there's no enforcement or true guidance of any kind. It's just like looking it up on the wiki, having it printed out, having a physical checklist, or reading the forum post. For informal challenges, or those players that can easily restrict themselves? Sure Simple Achievements works wonderfully. It's that slight bit more convenient for a lot of people than the other methods.
The issue for me is that I gain nothing by using it. At all. I would rather have a printout than SA any day of the week. If I'm installing a mod (or mods) to help manage the challenge? It should be capable of doing things I couldn't do with a pencil and some scratch paper, in my honest opinion.
Things like Progression, Eureka, and HQM allow me to ENFORCE stages. They let me treat them as rules of the game much like "you can't jump more than 1.375 blocks without a jump boost effect" (At least I think that's the jump height... I don't want to go grab snow layers to check right now, and the point stands no matter the actual number). This is added capability, and IMHO that's what is required.
Especially when assembling a modpack specifically for a challenge, you should (if possible) have a mod that enforces this. Because you're going to get the average player using the pack, and nobody is perfect. I prefer actually not being able to space-cadet the rules. Like the Refugee to Regent Utilities mod by
@Senseidragon. I can't forget that I'm not allowed to go underground without armor, because his mod makes it obvious when I do. If I were doing the Tree Spirit Challenge, a similar mod could drain health if I'm not standing on my tree. Similarly a mod like Progression can enforce the tech tree limitations; which are the single biggest draw for me, they're the carrot leading me through becoming a better builder. Getting access to the better tech is my driving force in this challenge, anything that actually enforces that is a good thing for me. Which is why I'm at least slightly against SA. It doesn't enforce, so why should I care when I COULD have enforcement?
I mean without the mod integration (which is nearly entirely tech restrictions), the challenges produced by
@Maul_Junior and
@Monarch_of_Gold might as well just be tiny (or maybe not so tiny) flavor expansions or additional restrictions on Iamchris27's original City Construction Challenge. The mod integration is really what sets these apart, because the authors are taking into account the massive changes that come when you allow mods. The horizons expand at a breakneck pace when you add the community content of which there is orders of MAGNITUDE more than vanilla, the interpretation of which without guidelines is something which turned me off from a lot of vanilla challenges; I can't play vanilla anymore, haven't been able to since 1.2.5, Mojang can't produce content at a quality level and speed that can satisfy me. Which is why I'm still peeved at the lack of the mod/plugin API that would open up the ability to take the modpack launchers to the next level, so even "vanilla" players could start experiencing the expanded content from mods. Mojang could focus on improving the bones of MC, and we'd quit getting updates that half the players couldn't care less about, and maybe get some bug fixes for issues that have been outstanding for over 9 major versions.
Anyway, where was that thought going? I honestly can't recall, but if it works for YOU? Go for using SA. Personally I would suggest if you're going to be making a pack? Try to get as much packside enforcement of important rules as you can, because players WILL cheat the system, often unintentionally, and that sucks IMO. I did it a lot with early versions of Agrarian Skies and Crash Landing, and it sucks when the author puts a lot of effort into making something work a certain way, and I know a mod interaction that lets me get unlimited free water in CL, or I skip entire quest trees in Ag Skies. The same thing happened in R2R when I played it a long time ago... I did things and then had to destroy items or rebuild buildings because I forgot the rules and screwed up. Mod/Pack based enforcement would have taken a lot of that burden off of me, freeing me up to build things instead of count a bunch and trying to remember the support rules, or go into a cave and have the GAME tell me when I've delved too deep. That aids in immersion and takes an amazing amount of cognitive load off of me. Simply Achievements? It can't do that in my estimation.
But seriously, if it works FOR YOU I'm not going to tell you to stop using it. Everyone works differently, and if something works for you, more power to you!