Add to this the fact that cloud-based UC provides the same redundancy we have come to expect from a cloud service (think Amazon EC2 or OpSource), and it looks like in-house solutions are not even playing on the same field. And this is before you factor that UC in the cloud offers greater geo-diversity. With more companies turning to dispersed staffing systems as well as remote consultants/freelancers, the ability to make sure all employees have access to communication solutions is imperative while the inherent redundancy of the cloud increases reliability and prevents data loss during outages.
There are, however, a few things to be aware of before committing.
First is that cloud-based UC services are offered as "all or nothing." It is usually difficult and expensive (if possible at all) to bring your current e-mail and/or phone service to a cloud provider and "mix and match" these technologies with new cloud services for a hybrid UC approach.
In a way, it makes sense that cloud providers will not integrate with pre-existing in-house technologies -- they've already done the heavy lifting by creating an integrated UC bundle that works perfectly with industry "best of breed" technologies. Why would they jeopardize their offering just so you can keep your current business VoIP solution?
Despite this, the good news is that most providers offering cloud-based UC offer the widely accepted standards of Exchange and SharePoint, so it's not like your organization would be switching to an unproven technology for UC.
Secondly, outages can still be an issue as they are for any service. The cloud is not infallible, as Google's downtime experiences demonstrate. In theory, with a cloud-based solution a company should never be totally out of the communications game due to the sheer number of separate functions (if e-mail is down, VoIP may still be up and so on). The only "cloud killer" would be if Internet connectivity is lost - there's just no way around that.
Another concern around UC-in-the-cloud is bandwidth. Companies need to have fat pipes so all of the services function properly at the same time. The bigger the bandwidth, the better. Bandwidth not only helps speed up the delivery of the UC services, but can also combat some common problems with VoIP, like "tinny" voices.
Bandwidth is a serious consideration when it comes to UC in the cloud, so if your Internet provider is unreliable or struggles with high data loads, you should consider investing in a faster Internet circuit, implementing a QoS router, or other alternatives to cloud-based UC.