1. The first year: People say your 1st year doesn't matter, but it definitely does. If you don't get staffed quickly and in a meaningful role (easier said than done), you'll be behind as soon as you leave Core Analyst School. ACN is extremely competitive, and you will get stomped if you don't come flying out of the blocks.
2. Corporate Culture: ACN pays lip service to work-life balance, diversity, etc., but in reality we propagate a homogeneous, highly normalized culture that ostracizes anyone who doesn't ascribe to it. It's not enough to work long hours; you have to "go the extra mile" and do things beyond your role description to be at par with your peers. The median is then set higher than it is in other environments, so overachievement and self-sacrifice become an expectation at ACN. Also, if you're an experienced hire (especially a seasoned vet), think very carefully before coming over to ACN, as the culture is set up to benefit the "homegrown" folks who have been with the firm for their entire professional lives.
3. Talent management: My bachelor's is in Org Behavior/Management, but I was aligned to a systems integration practice as soon as I was hired. We have a practice called Talent & Organizational Performance, but since I started out aligned to a different workgroup, I couldn't gain experience in that space (even when there were available projects/roles) and now make a living plinking away at software and managing implementations. HR's red tape can be quite resilient when trying to move between workgroups. Hopefully, new analysts will be able to choose their interests more easily than we could a few years ago, but I don't see much changing culturally at ACN in that regard.
4. Promotion processes: This is a major gripe that many current employees have about ACN, and is a reason why we've lost a lot of good talent (especially lately). Our performance management process is, in short, a mockery. I've seen people get rated in the top 10% of their peer group get held back from promotion because someone on the ratings call didn't think they participated in enough community events, or because they turned in an expense envelope late. For a firm that prides itself on processes, metrics, and hard evidence, this is asinine. If ACN doesn't improve this process, people are going to continue to be demoralized, making them more likely to jet once a better offer comes along.
5. Travel: I also listed this as a pro, but it also has huge downsides. If you are married or in a serious relationship, good luck. It takes an enormous effort just to maintain your relationship on the road, and that's irrespective of the many stressors you'll encounter at an ACN project on a daily basis. Even if you're single, your physical and emotional health can take a big hit just by virtue of the wear and tear on your body.