Before anyone believes that virtual appliances are all positive, it is important to note there are significant problems as well. For example:
Coverage. There are only a limited number of virtual appliances, and their range of coverage of features, functions, platforms, applications, and scope is nowhere near the vast range of choice offered by software solutions. This makes the choice of virtual appliances somewhat limiting.
Security and Risk. Despite having some security advantages, the closed environment of a virtual appliance is also difficult to protect from malware and intrusion. Patching and configuration is obscured (and sometimes not even possible) and virtual appliances rarely come with sufficient security agents and toolsets (virus scanners, intrusion detection, firewall, malware detection, etc.).
Support. Just like other open source code, virtual appliances generally contain a complex (and often undisclosed) mixture of software and frequently lack accountability for support when problems happen. The demarcation between suppliers is more complex, and users have no single vendor contact, and no clarity as to what components are involved.
Virtual appliances do benefit from many of the same positive aspects as physical appliances. However, they also suffer from many of the same problems as both software solutions and physical appliances, as well as a few additional problems that are unique to virtual appliances.
Like software solutions, they still need the additional cost of the operating environment and infrastructure, including hardware, software, licenses, etc. And they can easily suffer from compatibility issues with other systems and applications running on the same box.
Like physical appliances, they are hard to maintain, do not provide open interfaces, and provide no ability to monitor or manage with standard system management agents. They are similarly difficult to secure adequately, and in fact are at risk from specific additional security exposures introduced by virtualization technologies.
While virtual appliances seem like a very good concept, in most instances they actually suffer from the worst problems of both software and appliances, and are really not yet mature enough for mainstream production use.
Hopefully they will mature as a market, and overcome these significant problems, because they show great promise. In the meantime EMA recommends enterprises seek to take advantage of their benefits for trial, demonstration, and proof-of-concept purposes only.
Andi Mann is a senior analyst with Enterprise Management Associates, an independent industry analyst and consulting firm dedicated to the IT Management market.