I think people should be aware of what changing to QCS as the connection method means, so I'm posting this.
When the problems with deleted accounts popped up on both Quicken sync to Mobile/Web and Express Web Connect when using the new QCS (Quicken Connection Service {also known as Quicken Cloud Sync}) it was clear that they were both using the same system. What wasn't clear to me was if they were both syncing to the same exact "Quicken cloud account data location".
I have done a test and the answer is yes they are exactly the same.
To demonstrate I will start out with a data file that is setup for Quicken Mobile/Web sync, which a PayPal account that is Express Web Connect/QCS. Here it is Quicken register on top, Quicken web on the bottom showing the last transaction after a "cloud sync" has been done (if you right click on the image and open it in another tab you will see it full size).

Now I enter a transaction manually.

Then follow this with a One Step Update that does the transactions update, but not the "cloud sync".

If these are going to separate locations I wouldn't expect it to show up on Quicken Web.
So refresh Quicken Web and here is what we see:

So without doing a "cloud sync" the transaction was synced to the same location used by Quicken Web/Mobile. Clearly they are one in the same.
Note also this sync is by directional, and as noted other threads that has ramifications like image attachments will be transferred to the Quicken server, possible damage to Quicken Desktop data file when the data file and the Quicken cloud account data get out of sync (deleted accounts, reconciled transactions becoming not reconciled, ...)
Now lets say you don't turn on "Quicken cloud sync" what does that really mean for this data?
It means that the account are marked "not visible remotely" or something like that and that data isn't transferred to the Mobile/Web applications. The same would be true of "not syncing XXX account".