While it comes up, I think it's more of a rare problem. So much stuff is "x86 linux" or in rare cases "ARM linux" that it doesn't often make sense to have a cross platform CI system.
Obviously a db is a counter example. So is node or a compiler.
But at least from my experience, a huge number of apps are simply REST/CRUD targeting a homogeneous architecture.
Unless we're talking proprietary software deployed to only one environment, or something really trivial, it's still totally worth testing other environments / architectures.
You'll find dependency compilation issues, path case issues, reserved name usage, assumptions about filesystem layout, etc. which break the code outside of Linux x86.
Obviously a db is a counter example. So is node or a compiler.
But at least from my experience, a huge number of apps are simply REST/CRUD targeting a homogeneous architecture.