This can be a limitation on technology but also is a limitation on leverage to obtain premium vendor pricing discounts in the future.
Take, as an example, a shop that wishes to run HP Integrity blades with their Intel Itanium processors today. They invest in blade enclosures and begin using them. In three years they purchase software that runs on Sun UltraSparc or IBM Power processors. In order to use blades each of these technologies will require their own brand of blade enclosure and will significantly increase the risk in a small shop that enclosures will not be able to be fully populated.
There is much more flexibility in technologies using traditional rackmount servers because each vendor generally supplies one set of RISC or EPIC-based systems and one set of AMD / Intel-based commodity systems. If you want more than that, blades will become quite difficult for a small shop to manage. I have worked firsthand with shops that use multiple technologies like this on a regular basis, making blades a most difficult choice today before considering potential future platform decisions.
The fourth danger is the shared backplane and other key components. A blade enclosure, while generally built with massive redundancy and with truly amazing design, still represents a single point of failure that must be considered.
If your enclosure fails you do not lose just a single server but as many as sixteen physical server platforms. With rackmount servers you can add redundancy simply by adding an additional server - typically one matching server for each server you need. With blades you have to have redundant enclosures for the same level of reliability.
Again, for a large enterprise this is trivial and obvious. For a small business the need to suddenly own dual enclosures for full redundancy will often result in simply foregoing that level of protection and increasing risk.
The fifth danger is in the cost of flexibility. Small IT shops may not often move their equipment around. The option is generally there, though. If a small business owns three servers and replaces one with a shiny, new unit the option is almost always there to redeploy the old server to another role - perhaps in a branch office.
With blades the old blades can only be redeployed in a location that has a blade enclosure matching the one from which the blade was pulled. This is a cost of lost opportunity late in the server lifecycle and often completely ignored in cost analysis of blades. If there is not a spot ready for an older server it is far more likely to be discarded in the blade model rather than redeployed. This is true unless the company is large enough to have many enclosures available all of the same generation and with available space ready to accept an older server.
The sixth danger of blades is the high cost of storage. Storage is a subject all its own these days with SAN, NAS and DAS as possible options. Shops of all sizes are moving to SAN and NAS quickly and with enough network storage in place this can alleviate much of the storage risk associated with blade servers.
Many shops, however, use circular reasoning and justify SAN because of the blades and blades because of the SAN. Taking a holistic view of the server and storage picture is crucial.
A typical blade server can house only one or two 2.5" SAS or SATA drives. This is far less than a typical rackmount server would provide as potential storage space.
It is common to find eight to sixteen drive bays available in popular 2U rackmount configurations - sometimes using 3.5" drives rather than 2.5" drives.
One popular and very cost effective 2U server can hold 28TB of low-cost storage on fourteen spindles. You cannot put this type of storage into a blade enclosure. Because local drive space is simply not available, blade server owners are forced to use minimal direct attached storage and use SAN or NAS instead, even when DAS would provide better performance and cost (otherwise) for that particular application.
To bridge this need most blade vendors provide storage blades - blade servers that act as tiny, low volume SAN devices and fit directly into the blade enclosure. These units are generally of rather low capacity, often just six drives, and rather expensive compared to other means of providing storage.