ok, I'll attempt to respond in reverse order:
The ITAM 2016 Survey indicates time consumption with some device management requiremnet, not necessarily data access management. that said, anybody spending 4 hours a week chasing AWOL and new devices has a methodology issue.... not a technical issue ( go see Rory at SamCharter!)
Ian: Your response has a few layers; I'll start at the top. From a SAM TOOL POV, any changes/upgrade in the external data source ( such as SCCM) are typically understood and dealt with by the SAM TOOL vendor long in advance; many of these data source entities ( SCCM, ServiceNow, landesk, Lansweeper) has beta programs where all involved - including customers - are aware of database changes that could affect ODBC, SQL, API connections.
So that's 'configuration change'; the other issues ( firewall, system outage, site outage) are systemic issues that affect a company's digital infrastructure. When these things happen ( and they do) , they are outside the control of the SAM Manager - or even the company ( think about when the Microsoft Azure system - or Google Gmail - went down).
Keep in mind that SAM operates more from a governance POV, not obtaining device data from SCCM at 2pm on Wednesday doesn't shut down the SAM operations... you can get it at 4pm.
Also understand that such data fetches do not have to be dynamic and persistant; the customers that use AssetLabs web service have varying frequencies based upon the device usage type ( Hi churn desktops versus low change servers) and the churn rate within the technology types. With that in mind, SAM operations may not be affected by punctuations in the network/internet! ( An accident on the highway is only an issue when you're on the highway.... not while at home or work).
As far as complex organizations taking *years* top deploy tooling; I would have to say that you've just experienced the 'Game of Thrones' issue , where politics and bureaucracy of a 'kingdom within a larger empire' is the actual throttle issue.... not the technology. Classic 19770-1 approach needed there.
These issues of data collection are not exclusive to a SAM Tool vendor; they are also experienced by SAM managers & consultancies who attempt to harvest exactly the same data using manual and quasi-automated methods (scripts, ODBC connections).
Lastly, you've stumbled upon the reason why SAM has not systemically matured over it's commercial inception in the 90's, or why there is no real 'Silverback' solution provider in the market today: SAM is more of a Governance strategy that is dependant upon many heterogeneous Management solutions, protocols and policies.