Kraang, Utroms and vulnerability
 

I actually never understood why Krang or the Kraang are so vulnerable to attack. Krang himself doesn't have protection. I know the toy versions have glass protection on the stomach area which he should have had in the cartoon. Otherwise, the Turtles just need to hit Krang in the stomach area, which would really be punching him in the face. The Kraang also have that problem, too. With their stomachs being wide open. And Kraang Subprime is even more vulnerable as he has no protection from the front or behind.

The most notable Utrom that is totally protected is Chrell and he's completely covered. Strangely, Krang has a glass shield on his bubble walker, but not on his more powerful Android body. I don't think there's any Utrom or Kraang more vulnerable than Subprime. It seems like it'd be easy to take down the Kraang or Krang.

The simple answer is they're kids shows and they can't show the Turtles directly impaling the Kraang/Utroms in the faces with their blades.

No.

But they can show them punching or kicking them in the face. Or just pull them out of their android bodies, like Karai did to Krang in TF or what 2012 Leonardo did to Kraang Subprime in TT.

CyberCubed basically put it straight...though there were occasions of them taking direct hits to the face. A big one being when Splinter shot an arrow into Ch'rell's face. Which in all honesty, should have killed him, but he just pulls it right out. So the other question is, what in the world are the "bone" parts of Utroms made of?

The turtles had attempted striking Krang face-on, but he had glass protection and would only go so far as making the glass crack.

Donatello and Michaelangelo could hit with bō respective nunchucks.

So far as I know, I've only seen the OT Turtles knock that android body onto its backside in at least a couple of the cartoons - that robot wasn't always that good at getting back onto its feet. In Leonardo Is Missing and in the The Fifth Turtle, once that robot body went down, it needed help getting back up.

It was funny to note that it managed to get back up after it got knocked down by Raphael in that Mutagen Man episode.

As for that glass protection breaking, I never saw that, nor have I seen Krang take any blows to his face in the cartoon.

As for any of the Utroms getting ripped out of their robotic bodies, I see more of that in Nick TMNT.

I dunno ...cartilage?

It was able to withstand an arrow fully impaling him on the spot, so it had to be something a little more than cartilage. Clearly, it still hurt like hell from the way he reacted to its impact, but it still should have killed him.

Whatever it is, it's still not strong enough to resist being crushed. Several Kraang have been shown getting stomped on (one by Zog), and an Utrom in the Krang Mini by Traxus - someone not nearly as big and heavy as a Tricerton.

And to answer the "why aren't they being impaled?" Leatherhead impaled one while he was trying to escape capture in Dimension X. With a Stalagmite. Right through the robot it went. And not a drop of blood to be seen? I'm calling tripe on that.

Extraterrestrials
 

But the tolerance is usually higher for extraterrestrials than humans or other Earthlings.