As you know the fridge cools the air, then you need the air to cool the liquid, then the liquid cools the font.
It's often the case the customers do not have a true "flooded font too" and often they just use a glycol loop with another air cavity between the glycol lines and the air itself around the glycol loop.
Each time you change from one media to the next and go from air to glycol the temp increases. For instance if your kegerator is set to 2C for example and the ambient temp is 25C then you will get fairly poor results. For example lets say your glycol tank and the font on the kegerator have a similar surface area and are similarly conductive (both made of stainless steel) then the best case scenario is that your glycol would be half way between ambient temp and the air temp. So the glycol would be about 12.5C. At that temperature you would actually be warming your beer as it goes to the taps not cooling it down.
If the fridge is air cooled and you have enough space in the font to blow air you would still be better off just blowing the air directly up the font.
I should also say a properly made fully flooded font absorbs a load of heat too. For example this flooded font here will absorb about 200 watts of heat when it's sitting at -1C
https://kegland.com.au/products/4-tap-t-bar-flooded-font-stainless-steel
Just to put that in perspective the total cooling power of a small kegerator would also be about 200 watts of cooling. So your fridge would be running at 100% duty cycle just to keep the font cold let alone cooling the kegs. It would really struggle to be honest.
If you want to do a flooded font properly you should get a much larger chiller that is rated to 500watts or more so you sill have sufficient wattage to cool beer as well as the font while getting the font down to -1C.
For almost all domestic customers simply improving the insulation to your font is time better spent and then air cooling as mentioned above.