Combing and Coasting
Here is my understanding of Coasting: Coasting stops extruding early in a move so that the string itself will finish the layer.
Here is my understanding of Combing: Combing reduces the need to retract during travel moves by making sure that the nozzle oozes where you want it to on the way to the next point.
I'm curious as to what types of prints these are good for, and also what types of prints these would be bad for.
So for instance, Coasting is good for prints that have a high propensity to exhibit stringing, but what types of prints would I want coasting to be deselected for?
Similarly for combing, although I know neither the pros nor cons other than it reduces the number of retractions (decreases wear on extruder?)
In short, basically I'm looking for the pros and cons of both of these settings. Also if my understanding of the settings themselves is incorrect please let me know. Any advice would be much appreciated.
If anything, combing and coasting allow to mitigate problems that are printer and filament specific, rather than dependent on particular STL models.
Combing helps - as you imply in your question - with materials prone to oozing (e.g. PETG)
Coasting is particularly good for printers with a bowden extruders and low jerk/retraction speeds. This is because in bowden extruders there is a lot of filament compressed between the teeth of the extruder servo and the nozzle, and that pressure doesn't instantly disappears when the printer stop "pushing" (i.e.: turning the extruder servo).
I believe there are firmware implementations where coasting is also used when approaching sharp corners. This is to mitigate the problem of "blobs" forming there. The mechanics of this are similar to those explained above: the pressure within the extruder cannot be instantly relieved and coasting accounts for that. The only difference being that - because of the micro-scale of the problem - even non-bowden printers are prone to corner blobs.
In my experience (I look forward to other answers to "compare notes") there are very few reasons not to use combing. The only risk with it is that it increases the risk of the nozzle crashing into the print and destroying it. It sound dramatic, but it is in practice it requires everything to work against you: a big blob on the previous layer, the nozzle passing exactly there, poor bed adhesion... for me that has proved problematic only when printing miniatures with a 0.2 mm nozzle and 0.05 mm layer height (on a cheap printer).
There is of course a (usually very small) time penalty in combing, as it typically requires the nozzle to travel longer paths.
In my experience (again: YMMV, I look forward to more answers!) the limitations of coasting are related to the way it is implemented. For example, a given coasting setting may work great for getting rid of oozing, but will create under-extrusion in other parts of the print, as the calculations performed within the firmware may be spot-on for linear motion but inaccurate for corners, or vice-versa.
I believe this is the reason while some popular slicers (like cura) have this setting hidden under "experimental".