You cannot work on an iPhone with a soldering iron, it takes crazy expensive hot air re-flow stations to work on those PCBs, usually that stuff isn't even worth working on.
The only thing that can't be done with an iron, the right materials, and skills, are BGA packages.
Fine pin packages can be done either one pin at a time, or better IMHO, via something called the drag method (good flux and enough of it is critical). In some cases, a microscope may be of assistance, but it is actually possible without being forced to use hot air in many cases.
Youtube has how-to videos if you're interested.
Hot air stations are nice however, and are much easier to use for removal (and work with BGA's too). Not all are expensive though (Chinese imports that do work fairly well, and can be had for $100 - $300, depending on vendor and features).
You also cannot solder the older larger wired stuff with a soldering iron either. Think watts.
If you try, it'll take forever and you risk cold solder joints.
It's possible to heat heavy gauge wire sufficiently with an iron.
And it's not the watts, but if the iron can deliver enough heat to the joint (larger irons are typically used, but it comes down to the efficiency of the heating system, tip geometry & mass, and regulation).
Take a look at JBC's demo videos for example, and you'd be amazed (i.e. soldering two pennies together with a small iron in a couple of seconds).
Haha I just used hakko as my example because that is what I have the most experience with and it's affordable. I have no doubt that there is much better to be had out there.
936/7 models were built like tanks, and well priced, thus generating a well deserved reputation IMHO.
