I can totally attest. I worked on a typeface (Berkeley Mono ) for better half a decade. Have 1 variable axis (wght) and only 4 fixed cuts, approx 450 glyphs.
It’s one of those things that I can only continue to work on because of an internal drive and personal passion/enjoyment; otherwise it requires a lot of time and funding. Google and Apple can do that.
For those who haven’t delved into typography, changes that you make to a single glyph often has cascading and recursive effects to the entire 450 set of glyphs. Change the shoulder of ‘n’? Now, you must go and update similar glyphs for cohesiveness u, h, o, p, q, m, etc. Now the fitting is messed up. So change 100 more glyphs. Oh wait, the entire cut looks off. Should I proceed and move on, hope for optical satisfaction in future? Or is this the wrong hill to die on and undo everything? There is a lot of automation involved and wrote a ton of code, use components, plugins, etc. to automate as much as possible. The problem with typeface design is the toxic (or magical!) mix of subjectivity and objectivity. Some things just has to be optically evaluated and have to trust what your eyes lie.
Needless to say, Roboto is one of a kind project and insane amount of work. Variable axes require a tremendous amount of testing, often non-linear interpolation and endless adjustments.