I like how you can multiquote out of order. 
Me finding Jack useless probably has to do with me using my squadmates as pure defense strippers, staggerers (Zaeed, who can stagger/panick entire groups) or detonaters (Thane). Jack does none of that.
And I don't find her ammo power as useful as Jacob's. (which can also panick)
Every squadmate has their role. She isn't a defense stripper, so if that's what you are looking for, Jack isn't for you. 
Well, let's think about this.
For an Engineer, you want to rank up your passive as a priority, but a rank 1 drone will distract as effectively as a rank 4 drone. I usually get drone to rank two so I can unlock the next power in the tree (I can't remember which). Other than that, I am looking at incinerate and overload, depending on the mission. Rank 2 overload has the stun effect on synthetics, so that is useful. (Does it actually stun Geth? I didn't see it stunning Geth when I last did Haestrom.)
For a Soldier, AR is obviously a priority, but the passive leaves a bit to be desired. Squad Cryo and Inferno Ammo seem to be the priorities since they will do more for you than your passive.
That leaves the Sentinel, Adept and Vanguard who are focused on both their signature power and passive.
I actually find Soldier passive to be quite beneficial. The increased storm speed and duration of HAR really helps you move around.Also, I think soldiers tend to work more in the health pool than other classes, so the health bonus is relatively more valuable (I personally find the health damage animation very annoying).
Yep I am with a_mouse on the passive. Storm speed bonus is pretty big, and part of why I prefer Commando over Shock Trooper for Soldier. Throw on the speed bonus greaves and you are very fast, even out of Adrenaline Rush. Also agree about the health, especially if you are going Hardened Adrenaline Rush.
The point I was getting at was more about the strength of various characters in the early game via different upgrade paths. I think I could make a case that if I ran trademark power, passive, and then a one or two point power here or there on most of the classes it would be superior to that sort of build on Infiltrator (although some depends on bonus). This is almost certainly true for Sentinel and Vanguard.
I don't disagree that there are different ways to approach the different classes, but I would suggest there might be different build paths that make some of the harder plot missions go more smoothly, although that is still somewhat subject to mission order and squad preference.
One of the nice things about Soldier's ammo order is it miraculously progresses exactly as you need it in a typical run. On Lazarus or certainly by Freedom's Progress you should be able to have at least Disruptor 2, which can stun the mechs. For Professor you only really need Incendiary 1 to stop regen. By this point you would have probably decided whether you are "rushing" to Hardened Adrenaline Rush or putting a few points in Combat Mastery for Storm and Health bonus. Even a 3021021 (bonus whatever, how about AP for pure class) would work out pretty well for Archangel. By Horizon you can get Hardened AR, DA2, IA1, Commando, Whatever Bonus 1 and do pretty respectably. Or you could have stopped at 3 in passive to get Cryo. Either way works really.
On Engineer it is an interesting choice. You can try to get a lot of powers, or you can focus on one or two and let the squad fill in the gaps. I just think it is interesting that perhaps maxing Incineration Blast on Infiltrator fairly quick with 1 pt Cryo Ammo is better than going deep into Cloak or even passive, and how that might seem close to an Engineer. Maybe it is the fact I like the Shuriken more than the Mantis early game (or rather I don't like the ammo capacity of the Mantis). Don't get me wrong, Mantis is great if you can keep ammo in it. Sometimes that is easier said than done (Warlord can get you here).
It partly comes down to bonus of course. I like Neural Shock on the Infiltrator because it fills in a perceived need early game, IMO, which is organic CC. Lets me fool around a little more with other talents before having to get Cryo or Incinerate necessarily. Of course on the flip side of that, the early missions have a fair amount of Blood Pack and Incinerate into Mantis makes more sense for some enemies than even a cloaked shot, depending on how high you can get Incinerate.
Anyway I picked Archangel and Horizon because they are both keyed as tough early game missions. Really has to do with how they stress the team. Archangel is the only mission with all 3 merc factions together and they have different defenses so it helps to be versatile. Horizon is probably supposed to teach you to strip defenses with weapons since it has a lot of enemies with Barrier and there is no AOE direct anti-barrier power available for it outside of one bonus power.