Elective is suicide in large empires.
I didn't want to develop carpal tunnel syndrome, so I had to create lots of duchies and kingdoms to minimize army organizing time to fight rebels and other such threats. When you make kingdoms and such, all you have to do is select one of their provinces to summon their entire levy. So you can easily create 10,000 strong armies in foreign lands. It's one of the reasons why I could expand so easily. I used the Islamic gilded cage system of imprisoning all the major kings so they couldn't really cause any trouble. However, my previous Emperor died so suddenly that I didn't realize I only imprisoned 3 kings this time.
Not to mention all the previous successions tended to go relatively well with only minor rebellions as they were all quite normal. Males, with good traits, excellent diplomacy and the massive prestige of the dynasty. This time, I had so many negative relationship traits and terrible diplomacy.
I could have save scummed to fix that, but that's not really fun.
Anyway, here's Norse Britannia a few years before the collapse. We conquered everything and I had about 8 kings under my control. There were many kingdoms I couldn't create simply because of the creation requirements. My main kingdom was Ireland but I sent all my retinue troops to blitzkrieg Constantinople so I could move my stuff there. That moving all my retinue troops is what really killed me.
Most of the misc conquests you see all over the place were from my various kingdoms and duchies going out and declaring wars. It was another of the reasons why we could expand so well. Very easy to grow when you've effectively got a dozen independent conquerors operating under your name. Almost the whole of Spain, Scandinavia and Lithuania were done by my vassals. I should have shown you a picture of the kingdoms so you can see just how varied the conquests were.