What is a significant disadvantage of using VLANs?

The significant disadvantage of using VLANs lies in management complexity. While VLANs are beneficial for segmenting networks and improving security and performance in many scenarios, implementing them often increases the complexity of the network management. This complexity arises from the need to configure and maintain multiple VLANs, each with its own settings, policies, and security measures. Network administrators must ensure that the VLAN configurations are correctly set up to effectively manage traffic and security across different segments of the network.

Additionally, troubleshooting issues within VLANs can be more challenging because it involves understanding the interactions between multiple segments and ensuring that policies are correctly applied. This means that while VLANs can enhance security by isolating traffic, the trade-off is the added burden on management and the possibility of misconfiguration leading to vulnerabilities.

In contrast, the other options, such as high security measures, reduced performance, and easy accessibility, do not accurately capture the primary disadvantage. VLANs do enhance security by limiting access to segmented networks, typically do not reduce performance if configured sensibly, and accessibility can vary based on VLAN assignment and network design strategies.