Yes, you can use NAS to replace server. But we do not suggest to use NAS to replace server.

First, on hardware side, server is more stable and reliable. If hardware failure occur, if you purchase the warranty, hardware vendor will help you to replace the parts onsite. But for NAS, if some parts failed except hard disk, they may request to you to send the hardware to repair center for repairing. It may take few days or weeks. Most of our client will not accept that down time and purchase a new one. It will take installation charge and time.

At software level, if you are using Windows OS, Windows server provide many tools to help you manage the network and workstation. For examples, "Group Policy" can help you deploy computer setting automatically. Active Directory can help you manage users and workstations, e.g. when you create a user in Active Directory, they can use the same account to access all workstations. On the other hand, when you disable user's account, they will not be allow to access the network. By using NAS, you will need to create users' account in NAS, then on workstation, you need to create another users' account. when user change password on workstation, it will NOT apply to NAS's account. It make the management job more complex, duplicate or non-manageable.

Moreover, many software support Windows Server, many backup software, ERP software can be installed to the server to support your daily operation. But for NAS, compare to Windows server, less software or application can be chosen. On the other word, most of the application or feature provided by NAS can be found or installed on Windows server too. But most of the application or software run on Windows Server cannot be installed on NAS.

In a more complex environment, a server cluster consists of a group of servers working simultaneously. Windows Server platform provide high availability services. If one of the server stopped, other server can take up the role at the same time. Your operation can be continue without problem.

Honestly, NAS is a cost effective equipment. For small scale network, the advantage of NAS is more attractive. It is small in size, easy to setup, lower cost, server grade security file system, many useful application provided. It is suitable for company with 10 staffs or below. But as your business keep growing, we will suggest you to use server in order to provide a more reliable file storage and a manageable computer network.