In general, there are only a few key areas of added risk, for a cloud-hosted system as opposed to a self-managed system. Some of these may apply to some industries, others less so.
- Compliance - some regulations / laws make it difficult (if at all possible) to be compliant when hosted in the cloud. Lately most cloud providers are making efforts to meet this challenge.
- Data migration - generally you won't know where your data is located, e.g. it may be moved out of the country. This may be relevant again to compliance, e.g. Data Protection Act in the EU. Again, many providers are now allowing you to enforce a specific region for your instances.
In my opinion, as long as the provider meets your core requirements, the above issues are not so important.
However, these next two will always remain as risks, that should be managed appropriately by each business according to their own tradeoffs:
- Insider access - while you (hopefully) have some control over your own employees, even the administrators - via employee vetting, and other non-technical / personal controls, you have no such control over the cloud provider's employees.
While these can be partially mitigated, either by technical controls (such as encryption, which will only make it harder to do you damage, not impossible), or legal controls (e.g. non-indemnification, etc though highly unlikely), this will remain a valid business risk, that some business would not want to accept.
- Availability - obviously, you have slightly higher probabilities of high levels of access, when your machines are on site. Remotely, you're dependant on too many levels - besides the cloud providers, there is the ISP, and etc.
Again, there is some mitigation, both technical and contractual (e.g. SLA with penalty clause), however as above this can still be a valid business risk.
So, to sum up the answer to your final question:
Would you consider services sold on someone else's cloud too risky?
A simpler question is: Is it more vulnerable?
That has an easy answer: Not really. As @Rook said, there are too many factors equally out of your control, even when you own the physical boxes. So you dont lose much that way.
However, "too risky" is dependant on each business' risk tolerance, and various other trade-offs.
Aside from compliance issues, the last too points I mentioned would be the stickiest issues to manage.