They don't stack exponentially, they stack multiplicatively, and the effect of chain strat is not that amazing. Look at the damage output graphs you'll see that the peaks aren't astronomically higher than the trophs, certainly not to the heights that you'd expect from exponential growth. 15% multiplicative crit chance is simply not that great. Let's put it this way; the sam's crit chance seems to be around 22-24% overall which means his crit damage multiplier is around 57%. 15% multiplicative crit chance gives him a damage multiplier of approximately 5.3% (while it is up, not overall). Yes, this 5.3% is multiplied by other damage buffs that are active at the time, but it takes a lot of multipliers to make a low number into a not low number.
The raid dps during the initial period of chain stratagem is approximately double that after it; if we generously double the input of chain stratagem from 5.3% to 10.6%, with it's uptime of 45 seconds out of 5:45, or 13%, that means you've got a whopping 1.37% damage from chain strat, for that sam - and that is with a few particularly generous assumptions.
I don't know what you mean about more consistent, it would take incredible bad luck to have lower uptime on balance than on chain strat.
