Is "marginally lethal" like being "slightly pregnant"??
or is it like "less lethal" which is only part of the term "less likely to be lethal" which gives a distorted and inaccurate impression?
Every bullet that is longer than it is wide will yaw, and eventually tumble after striking something. Where the differences come in is in how much, and how soon (in inches of travel through the target).
Some calibers and bullet designs are more stable plowing through tissue than others. And if the bullet doesn't reach its "upset point" inside the tissue, but after it exits, most folks assume it doesn't tumble, but it does, just not where it can be seen. And when that happens, most assume they don't tumble, instead of realizing they just haven't tumbled
yet.
Nosler partition bullets are famously effective at lower velocities, because the forward portion can be made "soft" to expand at low speeds while the reat portion retains full mass and "pushes" them to achieve good penetration.
I'd say that non expanding or low expanding bullets are not "marginally lethal" they absolutely do kill. They're not as efficient as expanding bullets, but that doesn't make them any less deadly.