It's a harass tool.  Get some scouts in behind enemy lines and destroy structures.  They either need to put up some hanger defenses to chase down your scouts or live with losing a lab every five minutes or so.  It's not bad if you actually have five to ten scouts running around, but if you only have one or two you won't destroy anything, just damage it.

So, overall, it's mediocre.  You either commit to using it extensively or don't research it at all.  Way better than Advent's martyrdom, in any case.

As for quickly destroying orbital structures so you can move on, you're best off by having some bomber squadrons in your carriers.  Cleans up structures quickly and cleanly.

That's true of almost every ability in the game.  The auto-cast AI isn't very good, so you really have to be ready to manage manually.  If you aren't ready to do that, the ability is largely worthless.

You need several scouts using explosives to make it lethal against the target structure.  You have to kill, otherwise it's not useful.  This means you need several scouts, and you need to coordinate their attak.  If you don't want to do this, don't bother with the ability.  If you do have the time and effort to do this management, it's a decent ability to use.

Very much so.  The AI is largely an incompetent opponent capable of only the most basic strategies.  It mismanages its resources and fleet and depends on a hefty resource cheat on higher difficulties to have any chance of beating a human player.  I've yet to see a RTS with a great AI. 

Auto-cast is just a symptom of a larger problem, and that is an AI that is incapable of even considering the larger picture.

I guess the real flaw of the AI is that it is unable to think strategically in regards to fleet actions. It only thinks in terms of "whats the biggest threat" "Do I have vision? If no send a unit" etc.

Just today for example the AI was massing up a fleet to siege one of my planets. This one was heavily defended with a fully upgraded starbase. Around the starbase were 2 hangars, 2 repair bays and 13 beam turrets. The AI started by sending single ships in to scout out my planet. At first these were scouts. But then it started sending single ships like cobalts and a javelis. They arrived alone and all of the strike craft would pounce on them. For some unfathomable reason the AI then sent in a level 7 Dunov alone with its supporting fleet sitting in place doing nothing! The Dunov then charged my starbase and met with so much beam spam it didn't survive 30 seconds.

The AI relatively smart when it comes to seperate tactical engagements but when it has to tie multiple tactical scenarions into a larger strategic plan it just falls flat.

Timed explosives is quite exceptional if you mixed a few scouts with your attacking fleet.  Use them to hit a few lightly guarded structures so the fleet can focus fire on the defences and quickly wipe out the damaged structures later.

To quickly wipe out a planet, bring seige frigates.

RoN was/is the greatest strategy game of our time (arguably the best RTS as well).