Sharing my thoughts based on my development experience with the Mobile SDK for last few years. Note - I have been developing Force.com mobile apps since the time when the Mobile SDK did not exist and you'd to pretty much write your own wrappers if you were doing any hybrid development or use tools such as zkSforce for native development.
Stability/ Support - In general, the best place to find support is the Mobile SDK github repos. I've often found bugs already filed for problems that I have faced. Else, if I have discovered a bug, I have filed it there myself. In general, I have received either an acknowledgement for a bug or a clarification on why Salesforce does not consider it as a bug. In general, 2.x versions are much more stable than the 1.x which were more evolutionary.
Frequency of updates - You can get first hand information for minor updates here - https://github.com/forcedotcom/SalesforceMobileSDK-iOS/releases Minor releases come out every 1-2 months but you for major releases 2.x you typically have 1-2 releases/ per year. It's taken nearly a year to go from Mobile SDK 1.x to 2.x
Security - If you go through the list of currently open issues on the Mobile SDK github repo, there's a lot of work ongoing in order to make the Smart Store more secure. There are certain limitations such as this - https://github.com/forcedotcom/SalesforceMobileSDK-iOS/issues/392 which would be nice to fix but Salesforce won't do it in the name of security.