Danger with soldering, as stated elsewhere by more experience ones than I, is that heat from soldering iron soaks into the cell and overheats the electrolyte (and other chemistry) just under the end you're soldering.
Very difficult to manage/limit the heat-soak from the soldering iron through the metal into the chemistry.
Spot-welding is much shorter-duration and more manageable from a heat soak-through point of view.
Some post-mortems on cells done both ways would be nice (thinking of vertical sections using a buzz-saw, with photos, to see visual effect of heat-soak with both methods.
Very difficult to manage/limit the heat-soak from the soldering iron through the metal into the chemistry.
Spot-welding is much shorter-duration and more manageable from a heat soak-through point of view.
Some post-mortems on cells done both ways would be nice (thinking of vertical sections using a buzz-saw, with photos, to see visual effect of heat-soak with both methods.