I probably am just not "getting it" how fixed temp tips would make a good general purpose soldering system.
It's pretty simple, actually. If you have a very tight "control loop", then the tip temperature will not change much, no matter what load you throw at it. The tip will stay at pretty much the same temperature all the time.
Now, the solder has a specific melting point. All you need is to get (a bit) above it to do your work. The issue with simple soldering stations is that you have a pretty big thermal mass between the actual tip and the sensing element, usually, with JBC being an exception here, of course. What that means is the following:
Usually the sensing element is closer to the heater than the tip. So once the station reaches the target temperature after power up, the tip itself is still a bit colder than that. Only after a while the temperatures equalize a bit better, but still the tip (being exposed to the air) is a tad colder.
Now you place the tip onto a solder joint. What happens is that the tip cools down rather quickly. Due to the thermal mass it takes a while for the sensing element to actually notice that. Once it does, it turns on the heater. Which itself has a lag, plus the thermal mass to the tip. Many stations try to remedy that by trying to maintain a higher heater temp than what you actually set for the tip. Which leads to overshoot.
In case of a large thermal mass at your solder joint things get really bad, since whatever the heater produces is basically instantly sucked away.
Metcal, with the fixed temperature tips, works on a completely different principle. In these tip cartridges you basically have a transformer, with the secondary shorted. However, that secondary is made of a special alloy which, once it reaches a certain temperature, ceases to "be a shorted winding". Since they use RF, all that is very small, tiny in fact. All you have is a tiny copper slug, which is the actual tip, cladded in that special material. Around that is a coil, a few turns only. So once it reaches temp, it ceases to consume energy. Once you draw heat from it, thus cooling it down, it starts to consume energy, heating it up again.
Since all that is so small in physical dimension, you have a virtually instant response.
There simply is no need to adjust the temp: the specified temp is always maintained within a very narrow window, no matter what. Of course, if you want to melt some plastics with your soldering iron's tip, it would be great to have the temp lowered for that, otherwise you just burn it away. But then, given the nasty and aggressive chemicals in plastics, i would rather not recommend to do that anyways
Attached are two images. One is a 2-Cent coin soldered to a bare PCB, that is, just copper-clad with 35µ copper. The next is the same, just viewed from the side. The last one shows the used solder tip in comparison. It's 0.4mm tip. Soldering the coin to the copper took less than a minute, most of that time was spent rotating the board with the coin on it around.
Try that with a regular station, while being able to solder fine SMD parts after that, without changing the temp setting
Greetings,
Chris