Apple support documents seem to suggest that the Mac's switch to Apple's proprietary cards may not be compatible with GPUs made by third-party companies.
In a session at WWDC 2020 focused on bringing applications that use Metal to the new architecture, Apple made it clear that Macs with an Apple card will include an Apple proprietary GPU.
In addition to moving to Apple GPUs in new Macs with Apple Silicon, it also continues to hint at what's to come. In another support document, Apple recommends not underestimating the integrated Apple GPU.
"Don't assume that a separate GPU results in better performance," says Apple. "The integrated GPU in Apple processors is optimized for high-performance graphics tasks."
Apple also recommends that developers include just-in-time (JIT) compiler port when converting their applications to the Apple Silicon architecture.
Always make a call before executing any instructions from your threads. Instruction caches are inconsistent with Apple Silicon data caches and unexpected results can be obtained if you execute the instructions without invalidating the caches, ”says a Just in Time (JIT) paper.