Testing is always good, but I think glide went through a lot of work to make sure the apps scale correctly when using supported built in features.
CSS is not supported and should be used at your own risk and should be tested no matter what. It all depends on how well you wrote the CSS. Some CSS classes differ on different devices, unless you are assured that you are using stable class names that are the same on multiple OS/Browsers. Also, if your CSS relies on specific pixel widths and heights, it may not scale well on larger devices. Using percentage instead of pixels may or may not imrove the scalability on other devices. It largely depends on what you are trying to manipulate. There’s a lot to consider when you start to manipulate a platform that wasn’t built and designed to be manipulated in such a way. Our use of CSS is used to forcefully manipulate the standardized and stable design of glide. I use it to some degree, but in a very limited capacity with the expectation that it could break at any time.
If you stay within the guidelines and specifications of how glide was designed and how it’s supposed to be used, I think you should be pretty safe assuming that it will work on a variety of devices, provided those devices fit within Glide’s minimum recommendations. If you start to add unsupported or experimental features, then you run the risk of compatibility issues. Glide was built from the ground up to scale across multiple different devices, browsers, os’s, screen sizes. It’s only when you start to go outside of those standardized features that you should really take the time to understand what you are doing and test accordingly.
For these reasons, I typically avoid giving recommendations for anything outside of Glide’s supported features. I will share experimental things I’ve done or things I personally use, but it’s enough for me to support my own experimentation, because I know who is using my apps, how they are used, how to maintain them, and what I need to do if an unsupported feature stops working.
All of my projects are personal projects or projects I’m deeply passionate about. That gives me the creative freedom to design apps how I want. It’s not a job for me. My expenses are my time, so I can’t advise on any actual real world financial costs. Only that if you stay within Glide’s built in feature set, then testing should be minimal. If you start to experiment with unsupported features, then testing is paramount depending on your intended audience.