My problem with auto-update feature started with my firewall.
As implemented, updates create a new folder with a version number, so I've to authorize manually every time it changes. Without folder exceptions, my current firewall / antivirus will detect executable changes and require new authorization anyway, that and the fact I try to avoid possibly unwanted changes / errors, leads me to both block and disable the auto-update.
Also, I don't know how Windows deals with it, specially WinSxS wich has a tendency to grow and grow...
Unfortunately, the option to disable it is hidden as a command line parameter. Would be nicer to leave such a control explicit on the advanced configurations.
In my opinion, a program that updates all the time is premature, either bloating or degrading or needs a more clearly defined target - Or all of those, wich is bad because Opera is great. The fact the makers wan't to push updates (by hiding the opt-out) concerns me.
Just to add my two bits.