Is there a way to adjust the underlined type default setting in Illustrator, in order to make the rule thicker without outlining the type?
Forget about that function. It creates pure ugliness and the one thing it's good for is to simulate a link in a web design.
To make it a little less ugly create a line behind the type.
In the Appearance panel, create a newFill and drag it below the Characters item.
Use Effect > Convert to Shape... to make it a Rectangle an move it down with Effect > Distort & Transform... > Transform.
Save it for later as a Graphic Style.
Quite a long time ago when Illustrator did not provide the Underline Text option at all, there had been acute discussions with trench warfares between supporters and detractors of that feature.
Often there were some plausible reasons to support it and some dogmatic reasons to dislike it.
To me, the real disappointment is that it once was introduced in an incredibly ungraceful manner without any adjustable settings (compare it to InDesign). And no improvements since then. Pretty weak.
Ton, in the past I often suggested some similar workarounds. One problem is that it only works with (unrotated) single line point type objects or single line area type objects. (Well, some further very wobbly workarounds may work sometimes with multiple line type objects).
Have you already found a way to apply it to separate words or strings in single or multiple point type or area type objects?
Kurt, it is a hack and has many limitations. You can add a new transform effect to rotate the type and line, but it may be just as easy to draw a line in places you want.
Yes, Ton, one may do some other workarounds to overcome the basic workarounds.
One may also just realize that Illustrator is pretty weak when it comes to flexibly underline text.
I agree, Illustrator has a poor implementation of underscore and strikethrough.
For a more flexible and creative way to handle these, InDesign is the better tool.
You can create interesting type character styles, when applied to text, you can add the textframe to a CC library and use it in Illustrator as a linked (smart) object.