In researching the related question How can I break items down into their components, I saw a complaint on Reddit to the effect that the extra, leftover materials get destroyed.

Why this is a common belief

Many people seem to believe this, and there's a good reason for that: it's confusing. And it's confusing for two reasons:

1. The composite item is removed from your character's inventory, but the leftover components are placed in the Workshop's inventory.
2. There's sometimes a delay before the leftover components reappear. If you want to speed up the process: go into Workshop mode, try to build something, and then exit. This appears to force the Workshop to update its inventory contents list.

Go, science!

I settled the question with science. I've tested this with various items a dozen or so times using the Armor Workbench, the Weapons Workbench, and the Workshop crafting menu.

The leftover components have always (so far) appeared in the Workshop inventory, but there's sometimes a delay.

Here's one of the experiments:

Experiment

• Remove all Lead from your inventory and the Workshop's inventory.
• Add two pencils to your inventory; each pencil has 1 Lead and 1 Wood.
• Go into the Wood Structure crafting menu—not to craft anything but just to check your Wood component count.
• Craft a Radiation Emitter, which among other things requires 2 Lead.

By creating the Emitter, we force the Workbench to dismantle the pencils, which liberates 2 Wood and 2 Lead.

If our Wood component count goes down, wood has been lost/destroyed. If it stays the same, our 2 "potential" units of Wood have simply been converted into 2 actual units of Wood.

Result

Our Wood component count remains unchanged. So unneeded materials are retained, just as though you'd broken the item down yourself.

This is all from my own observation and I don't have an official source for it. But since I play on survival and found a LOT of legendary enemies yet, they are kinda reliable.

A mutation in Fallout 4 causes the following things to happen to an enemy:

• It will regain their health completely
• It will get a damage boost of about ~50% (stronger enemies get more)
• Certain enemies get a damage reduction against the damage you used most against him before mutation
• It may get a random "perk" from a set list.

I could not find out the total amount of perks, but according to the stuff I've seen, this is what may happen:

• Green aura: The mob deals much more radiation damage per second . Seen on a legendary glowing one at lvl 16 and a legendary ghoul at lvl 15.
• Orange aura: The mob dealt additional DoT damage with each hit. Seen on various enemies past lvl 20. It seems to be seperated between nuclear DoT and "normal" DoT (maybe even poison DoT?), since, when I wear a power armor, some enemies which dealt huge DoT to me before barely dealt any DoT to me.
• Red aura: The mob gets a huge damage boost to the boost he already gets. A muted watchbot almost killed me within 2 seconds when he didn't even deal half the damage before. Seen on this Watchbot ~ lvl 24.
• Yellow aura: The mob has highly increased movementspeed. Seen on a Mr. Gutsy.
• Nothing: The mob doesn't get an aura. Therefore, I assume it doesn't get any special perks. Not sure though, maybe it does, but a weaker version. Maybe also: The stronger the aura, the stronger the perk.
• (Unsure!) Blue aura (???): Grants huge damage resistance to the resistance the mob already gets. Was curios, and it could survive 2 Fat Men. Seen only once on a radroach at lvl 26, seems to be extremely rare or I'm colorblind, I was quite tired when i encountered this.

This is all the information I could find out by now, maybe there is more, I'll update the answer once I know it.