Grot tanks are what happens when a bunch of gretchin have too many spare parts and not enough supervision. These ramshackle "tanks" are quite literally piles of scrap barely held together with duct tape and hope, and like a lot of ork technology, they're somehow still effective. Their armor might as well be tissue paper, being as light as a vehicle can be, but the sheer stupidity of their design means that any given hit might just blow off some redundant junk or over penetrate rather than causing any real damage.
To be fair however, the Grot Tanks are indeed based off the Tankettes in World War 2, which are like miniature baby tanks that could only house no more than two individuals, their main role was to act as a small, lightweight but fast armored support vehicle to help infantry. Unfortunately like the Grot Tanks, Tankettes had such shitty armor that any decent explosives would turn them inside out, so they got immediately phased out.
Grot tanks are armor 10 all around, but because of this scrap they get a 5++ save. Unfortunately, their engines are just as trash as their armor. Grot tanks have a random movement that might be as slow as two inches, so they don't reliably go where you need them. The few weapons they manage to attach to these junkpiles aren't exactly worthy of the name 'tank'. A single big shoota, rocket launcha, Grotzooka, or other ork heavy weapon is all they get. Still, they're dirt cheap and have BS3, giving them some potential to be an effective fighting force.
More like a rolling mini battleship, and can be scratch built from one. The grot megatank or mega tank is like five grot tanks smushed together into one large gretchin-infested monstrosity. As such, they represent the pinnacle of Gretchin tank technology, an "overpowering" war machine that drives all before it in a storm of scrap and destruction.
As these machines are hammered together out of junk, spare Mekboys and unbridled Grot enthusiasm, the Grot Mega Tank represents the culmination of the Gretchin battlefield madness that has become known as the
lazy punny cringeworthy awesome name of "Grotzkrieg."
Five Grotzooka turrets, three of which are twin-linked and each of which can independently target foes, and armor that doesn't fall down in a light breeze (but won't stand up to most real AT weapons). It's got some interesting tactical flexibility, but again its weapon choices are limited to pretty much what you can put on a Killa Kan.
In 8th Edition, the Grot Megatank is a pretty situational vehicle that is based more on a game of chance than it is a 'point-and-shoot' type of tank. With a wide variety of weapons and better stats than the normal Grot Tank, it's tougher and doesn't have to deal with morale, but its randomness is its downfall. Every Shooting phase, you need to roll a d6 - on a 1 you can't shoot at all, and on a 6 you gain +1 to hit, but all your guns can only target one unit. You would think that with that many guns, the Grots should be multi-firing at multiple targets at once. You can reroll with a CP, but it is still a risk.
To makes matters worse, it has random movement, which means that - combined with the shitty range of these guns (average range is between 18-24", you ain't hitting far) - it is far more preferable to roll your movement before you declare your advance, otherwise a bad movement roll means your mini land-battleship is unable to hit shit. At the very least, it could opt to mount a Wreckin' Ball in CC that hits on a 5+ and it can choose to load up 7 Rokkits per turn at BS4, which is better at shooting TEQ units than Tankbustas.