From the way you have described the classes, it is likely that you haven't played mercenary or smuggler at all, or at least most of the way through.
For example, the mercenary has the second wind perk, earned at level five, that recovers half of the mercenary's health (somewhere over 200 from my vague recollection) and half of their max energy, an amount of restoration that the tech simply cannot match, with a skill that doesn't scale with leveling up, leaving it all but useless against level 8 enemies. Not to mention how the tech's shield restore skills don't recover energy, something incredibly important that they have no way to recover. Energy is critical to defeating everything other than trash mobs, something both smugglers and mercenaries have ways to recover, but techs do not.
You dismissed the smuggler's aimed shot as a mere "damage boost" but what you don't seem to know is that that damage boost allows you to destroy any boss faster than both other classes can, while at the same time taking next to no damage due to your foe's blindness stacked with your dodge rate, that is at the very minimum (it can be increased even more with accessories) 30% higher than the other classes.
If your foe isn't blinded and you don't feel like spending a turn doing so, you can just use the second shot ability, which so handily stacks with shoot first, to let you get three shots off per turn. Combine this with how smugglers have the option to be immune to stun locks and you have a class that floats over everything, not a class that is the weakest of the three.
I've played both Smuggler and Tech through the Frost Wyrm (level 8, 40 all stats). Haven't touched Merc so I'm just analyzing him based on what the skills do on paper (I missed second wind, sorry). I can tell you that the Smuggler absolutely SUCKS in that particular fight, and also struggles heavily late in Myrellion leading to some "unwanted" game overs. Took something like 10 tries with the best stats/gear on the smuggler to kill the Frost Wyrm, looking for ideal RNG just to survive. The Tech had such an easy time with it that I was able to fight and win twice in a row simply keeping Deflector regeneration active at all times.
Tech also has a 50% reduced vulnerability on shields. I'm not entirely sure how that works mechanically, but it should make his shields worth more than any other classes health in spite of not benefiting from armor.
I'll also note that the Smugglers Stealth Field doesn't really feel like it does anything at all. Blind isn't all that effective either. Even combined I still end up getting hit at least half the time, and with the cost to maintain those two status effects in both turns and energy it just doesn't pay for itself even with aimed shots.
Merc might be better than either, but Tech is FAR easier and stronger than smuggler. That's something I've tested and experienced rather extensively. Even without using damage skills at all (ie Volley) Tech is simply better at both offense and defense, and the smuggler doesn't even have any real damage skills to compare when the Tech starts pulling out Volley himself (which of course hits 3 times with Second Shot, at a higher accuracy than what the Merc can manage).