Hey @spanners welcome to Quasar.
You will be in no way tied to use Stylus, it is true that the internal components use Stylus as CSS pre processor, but you can choose whatever you like for your Quasar app.
The docs explain how you can switch the pre processor on the example of SCSS.
Also, if somehow there are issues with Stylus that are not resolved upstream, I am certain that Quasar will adopt accordingly.
I can only recommend you to join our Discord chat at https://discordapp.com/invite/5TDhbDg where the community is way more active. From the forum you could get a false impression about how much people care for Quasar.
Hopefully I could resolve you concerns, otherwise I am glad to answer additional questions.