A NAS will use a network file protocol (SMB/NFS/AFP/SFTP etc) to access data rather than direct disk access, so the types of failures are different. Generally you don't really have to "eject" but disconnecting during a large transfer can cause incomplete writes.

The main risk with directly attached storage is that most kernels will do "buffered writes" where the data is written to memory before it's committed to disk. Yanking the drive before writes are synced properly will obviously cause data loss, so ejecting is always a good idea.

Generally, NAS is a bit safer for this type of storage because the protocols are built with the assumption that the network can and will be interrupted. As a result, things are a bit slower since you're dealing with network overhead. So, like everything, there are some trade-offs to be made.