Rolled copper lays horizontally. You grow plating on top and sideways. So the shear point is if you go down in the tunnel at the point where the vertical plating transits from rolled copper to the dielectric material. That is a fragile point.
Well... maybe. If we're going to talk about the mechanical interface, let's don't forget about the ductility of the metal. Even thin layers of copper have ductility, and can withstand some expansion, contraction and flexion. In fact, copper is one of the more ductile metals, and that's one of the reasons (along with its conductivity) that it's the preferred material for wires.
Basically, the interface between the "rolled" layer of copper and the plated layer isn't as fragile as I think you believe it to be; the process of plating includes a cleaning step where nearly all of the oxidation is removed from the "rolled" layer before it's plated. The result is a nearly continuous interface. Provided enough copper is plated, there's enough ductility to handle the mechanical interface as long as you're not beating the board to death or trying to fold it, which would indeed place large shear forces across the interface.
The only way this would actually become a problem is if the plating was too thin. Most quality manufacturers will plate enough material to obviate that in all but extreme cases. I think the only real problem is that the plating in the via is thinner, and has increased resistivity; the way to obviate that is either to fill the via with solder or use a Z-wire.
Most microcracks happen during reflow. Improperly conditioned boards, poor temperature control and the via's will popcorn or microcrack.
If you're seeing this, your boards weren't sufficiently plated. It simply shouldn't happen if the board is up to snuff. However -- I'll give you this: larger boards
would be more susceptible to it, and smaller boards would be
less susceptible to it. A lot depends on the substrate. FR-4 will behave differently than polyimide or PTFE. What we're talking about here is obviously FR-4, but I think you'd be far more likely to see issues with PTFE, and far less likely to see them with polyimide, because of the differences in the thermal expansion coefficient for those materials.
My bottom line on it is the same, though -- if you're seeing the sorts of problems you're describing here, you've made an engineering error (not specifying sufficient plating) or your PCB manufacturer has made a production error (not plating sufficient material or not seeding the vias properly for plating.)