What you'll want to do right away when it comes to making art with semigraphics is to figure out the strengths of the semigraphics in question. To give examples from my own end of things, the "shade" characters in the IBM code page 437 character set, the one that all of "ANSI art" is built on, give the appearance of the image being aliased, and work best on images with high levels of detail where the half-block characters would cause the detail to look too, well, blocky. On lower-detail images, however, shade characters would make images look too fuzzy to be readable unless used sparingly for shading (of course) or used for things that should look fuzzy (such as snow, clouds, or flamethrower blasts), whereas half-blocks can be used in a similar fashion as pixels to form the concrete structure of a figure (especially if you're using something like Moebius or MoebiusXBIN which has a half-block brush). The line-drawing characters, meanwhile, work best for mechanical designs where the straight lines and right angles would fit, rather than organic designs like veins that demand curves that are difficult to represent at low levels of detail without custom fonts geared towards representing them.

It's also important never to treat anything in a "one size fits all" fashion and adapt to whatever the font you're using throws at you. For example, while I mainly draw my ANSI art using an 8x14 font on account of having started out with ZZT, the majority of ANSI art in the wider "artscene" is done with IBM's 8x16 or 9x16 fonts. In making the 16-height fonts, IBM extended the bottom edges of the semigraphics by two pixels while keeping their orientation the same. This means the horizontal half-blocks will be uneven in size vertically, and the horizontal lines in the line-drawing characters will be off-center vertically. On top of that, IBM accomplished the 9-width fonts by duplicating the 8th column for the half-block and line-drawing characters, and drawing a blank column for everything else. Because of this, the shade characters in the 9-width font will have a blank column on the right side (as seen in my first Xenia piece), the vertical lines in line-drawing characters will be slightly off-center horizontally, and the vertical half-blocks will be uneven in size horizontally as well. If you're like me and started out working with the 8x14 font that has the block sizes relatively even and the lines centered, it may take a little getting used to the imperfections of the semigraphics in the 8x16 and 9x16 fonts. On the other hand, to the amazing artists whose work in the 9x16 font is posted on sites such as 16colo.rs, these imperfections may as well be invisible.

I have not worked frequently with semigraphics sets other than the classic box-drawing characters of IBM code page 437, but I hope this helps!