Bambu made it very easy for everyone.
Put a (Bambu) spool in the AMS, hit print and all works fine.
Things quickly start to differ once we deal with lots of supports and complex prints.
And if you want to use custom filaments all hell seems to break loose for some users.
The good news is that most, if not all of those problems you might encounter are indeed fixable.
Ever wondered why the default flow ratio is always 0.98 or 0.95 in most slicers, including Bambu’s?
A solid 1 would make sense if we assume the filament is consistent enough, which it is these days…
Thing is that once you extrude line by line and wall by wall the extruded filament faces restrictions.
Like on one side already having a wall loop and the filament only able to expand to the other side.
On top of that filament changes when extruded.
What makes all this checking and calibrating so frustrating and confusing for new users is that you can compensate for a lot of problems by making changes elsewhere.
Like so many users ‘fix’ a rough top layer by ironing…
If there is only a few flaws in some areas of your prints it is just too tempting to fix them by all means, expect by starting at base one…
For most printers this filament calibration is more than common, Bambu however tries to make us think we only need it for custom filaments.
And then we get calibration tools that sometimes completely fail.
Ever tried to calibrate a clear filament with those 9 patches ?
The often vital pressure advance is not even fully explained in the Bambu Wiki.
No surprise then that the pattern or line calibration is often misjudged.
So what are problems we can see that come down to a bad flow ratio- and/or k-factor calibration?
Those famous additions to walls where the bottom of a print ends…
While these are hard to fully remove they are a clear indicator once they are too visible.
Another tell tale sign for the k-factor is issues around corners, openings in walls and for fine details.
Perfect looking print but large sparse infill areas turn into messy poles and strings, the top layers starting just fine but ending with a mess?
Again calibration issues…
While sticking to the 0.2mm profiles and using a 0.4mm nozzle you can get away with a lot.
Bad looking seams get fixed with a scarf and changing the gap value…
That messy first and top layer fixed by adjusting the flow ratio for them…
Quite often all hell breaks loose when variable layer heights are used or when the user tries to print extra fine with a 0.2mm nozzle.
Don’t be fooled to think that your prints looking a bit rough with the 0.4mm nozzle that they will come out much better when trying a 0.2mm nozzle without proper calibration 
I found a lot of posts where users came with replies like “I already adjusted the flow ratio up and down but nothing helps”…
Messing around is not calibrating…
Why do you use PLA for the support interface when printing PETG and the other way around?
Silly question, of course because otherwise those supports are impossible to remove…
Truth is that I can print my supports without this nonsense unless I need to support large flat areas or require a near perfect finish.
Even PETG snaps right off.
Yes, for very pointy things I do increase the xy distance with PETG as it tends to smear a lot but that’s it…
Supports fused to the model are the best indicator that your calibration needs work - just saying…