The main reason why ERP implementations are challenging that it changes the way a company works. Not its business processes but the way the process is carried out.

Another challenging aspect is the amount of money, effort and time involved. Any approach, from big bang to phases, there are considerable amounts of resources required to make it happen.

Then you have reluctance from the employees. They are used to the current process that they carry out today. Implementing ERP means that they will have to come out of their comfort zones and learn (make effort) a new thing alias reluctance to change.

Once a company has decided to implement ERP, then the challenging thing is to manage the implementation. Most of the times, the teams over grow and result in too much of cost and at the other times the team is not sufficient to finish the task in target time. Both cases lead to loss of money and / or time for the company.

Once the company has a optimum sized team, then the challenging thing is to manage the team, delegation of tasks, keeping sub-teams in sync etc.

If the implementation runs for a significant period of time, then attrition also poses challenges. Any new team / business personnel will take some time to come up to the speed and most certainly will have different views than existing. Managing harmony within the teams is quite challenging.

All in all, its a big step for any company, it will definitely be challenging. If the challenge is tackled efficiently, you are in for a sweet reward.