Nicolas,
I'm just in the process of setting Meld up for use on a commercial site, and so far I've been very impressed by the ease of use.
I've not delved too much into anything complicated yet, and can't comment on performance as it's not live for general use yet; but I have set it up with a hybrid database setup.
I'm running MSSQL for my main Mura install, and have installed MySQL just to run Meld. (I'm hoping in the future I'll be able to have them both in MSSQL!).
As far as I can see, all the user management is done via Mura's API, and the Meld database just contains a reference to that user. Any data that is required by Meld for display purposes is duplicated in the Meld database, and when anything that is stored in both places is updated - from either end - it is automatically synced across.
(This is just my understanding of what's happening - I'm sure someone will correct me if I've got it wrong!)
I'm very happy so far - and any problems/bugs I've had have been answered/fixed very quickly by the Meld team...