Sometimes one of the mini krugs falls in to the terrain and then you can't hit it to finish it off, hurting your clear. Sometimes you actually have to walk away to be able to hit it.
Plants consume empowered auto attacks. This is especially bad for Nocturne. Consuming his passive is pretty bad. It will also generate stacks for Yi and Aatrox, Xin stacks on everything anyways, but not sure but probably does for Nasus. I'm okay if it gives Nasus stacks, wards do after all. I'm okay with them eating abilities like Leona Q (Talon's old Q). If we can make them like wards it won't eat Nocturne's passive and still give Nasus stacks. However, if you can also make it not eat abilities like Leona's Q that's fine too.
Repro steps. Go in to any game, attack a plant with an empowered attack. 100% rate.
As for krugs, it's entirely based on the RNG of where they land after you kill the small Krug. But it's 100% if they fall in the terrain. Happens about 8% of the time.