The most obvious point there is as follows: the edges of the portal, which would appear to be 2-dimensional lines, and how reality interacts with them. For instance, there are no conditions for preventing a portal cut, as described in the power itself, and I shouldn't need to explain how potentially broken that is. I will anyway. Let's take a hypothetical substance, Godstone, which is absolutely impervious to all usual forms of damage due to how dense and hard it is- crush it, cut it, throw it into a black hole, it's just fine- and put it halfway between those portals as they close. Can the portals cut it in half, if they can't be stopped from closing up? If they can, then they can cut literally anything, and the portals are secretly an unstoppable weapon with an impossibly thin edge.

Likewise, the edges of a portal have a defined hardness based on the substance they're placed upon. If they're connected to a fluid, i.e. a gas or liquid, then their edges are not solid and cannot be touched. So what happens if something or somebody, say our hypothetical Godstone, falls on to that edge, or if a portal placed on air moves into it at the edge? Again, there's the issue that they can cause an impossibly thin cut against literally anything, because one half of the substance is going to a different place entirely. Similarly, what if you "break" the edge of a portal on a solid surface? If it instantly closes, and cuts anything inside it in half, we go back to the problem from before - namely, that the portals are secretly 2-D blades that cannot be stopped by anything.

But that's not even the most destructive aspect of the portals. That award, according to a close friend of mine, goes to their ability to change size, and the reason that's true is because for any given portal or wormhole, point X must map to point X2 - that is, a given location on one portal must move to the equivalent location on the other portal, and on a 1:1 scale that just results in movement through space, "ooo - ooo". On a 1:2 scale, though, you end up with "ooo - o o o"; 1:3, you get "ooo - o o o", and so on. You'd think that would just cause the stuff passing through a mismatched portal set to just grow or shrink, and in a manner of speaking, that's correct - on the scale of quantum foam, at the Planck length, for every single individual point on the portal's "surface". A substance passing through the smaller portal and coming out the larger portal might "grow", in the sense that each individual point of the substance has moved away from each other point, resulting in the substance being completely and utterly vaporised at the most fundamental level possible.

Not calmly, either; the nature of quarks and the strong nuclear force means that you need a lot of energy relative to the scale to remove a quark from an atom, and tearing them all apart with no significant energy input just by moving through space represents a massive violation of the conservation of energy, to the point that it's more energetically favourable for reality to spontaneously generate new quarks to pair with the ones you've pulled apart rather than re-pairing them with their original quarks. You don't even get replicas of the original matter at that point. Considering how particle pairs work, you might even get antimatter, and I assume I don't need to explain why that's bad. And given the usual movement of air, the larger end of the portal would essentially explode, instantly and incredibly violently.

Oh, but that's not even getting involved with what happens in the opposite direction, large portal to small portal. Remember how the points in space get stretched apart going from small to large, at the smallest possible level of space? Yeah, that doesn't get better going in the opposite direction - compressing matter in that fashion would, at the very very least, instantly crush anything passing through into itself, most likely killing any living being or rock-based lifeform. More likely, however, and especially with more extreme size differences, is that that end would constantly produce a deadly, hyper-energised stream of compressed particles, destroying anything the portal is pointed towards - and that from just the air passing through. As I said, pass a living being through, they instantly get crushed into that stream of particles, instantly killing them and firing them off to kill even more things.

And, of course, we have the obvious conclusion to the "how far can we compress matter into itself?" conundrum - make the difference between the two portals big enough, and at the small end you get neutronium: pure neutron matter formed because the electrons and protons of the atoms passing through got forcibly smashed into neutrons; the stuff that makes up neutron stars; and ridiculously dense, to the point that enough would probably warp Earth's gravitational field until the whole planet got crushed into another neutron star. Make the difference even bigger than that, and eventually each individual fundamental particle will be compressed past its own Schwarzschild radius, i.e. the point at which it collapses into a singularity, thereby degenerating into a black hole. Let me just reiterate that plainly: the small portal, if small enough compared to the large portal, will spontaneously generate a black hole. Depending on how much matter is compressed in that fashion, the black hole may just explode and scorch the planet, or it may persist long enough to consume the entire planet. Both options are absurd, and extremely fatal for everybody on Earth.