Re the term "comfortable with." Good discussion. It is not standard phraseology and the problem with all those non-standard expressions is that they are subject to interpretation from both sides.
I cannot remember having heard it directly as a controller in my career but I would interpret it, as: "I know what I am doing" , but perhaps it was meant as, "I think we will be ok "
As a general aviation pilot I have many times heard people using sentences like: "it will be OK" or, "it will pass" more to reassure themselves or convince themselves that they were taking the right decision.
One accident I remember investigating long ago (1993) was the Contactair Dash-8 that crashed short of 28 in CDG. They were on ILS runway 27, which became blocked. The APP controller asked the pilot: "Can you make a side step to the left to the ILS runway 28?" The Captain replied: "Yes of course!"
That reply led the controller to believe the pilot was happy with the suggestion (i.e. avoiding a go-around) and did not gave further instructions/assistance.
In reality it was a complex maneuver, as the threshold of 28 is 2500m ahead of 27. They tried to capture the glide from below but ended crashing 1/2 NM before the threshold.
Lots of discussion at the time about the initial request and the interpretation by the controller of the "Yes of course" . It ended badly for him in court later.