They're a lot more inconvenient than people acknowledge
First of all there's only so many glyph papers you can carry (did I mention that paper can be easily damaged?)
If you run out of them you'll need either 1. a surface that can be drawn on or 2. a spell to summon more glyph papers. If you don't have either of them you're screwed
Plus there's a lot of glyph combos to memorize, some of which are complicated/look similar to each other. If you mess up and use the wrong glyphs/combos you're screwed
Also if you accidentally activate a glyph while it's still in your pocket you're screwed (unless you're lucky enough for it to be a light glyph)
The only reason Luz doesn't have any of these problems is probably just bc 1. she's practiced for like... months I think and 2. she has protagonist plot armor that gives her photographic memory of any glyph combo she learns