The 5090 does have a bandwidth advantage, but also must pass more data from main to GPU memory for any particular task. Comparisons of a 5090 to a DGX Spark at 273 GB/s are illustrative since the Spark uses unified memory similar to Apple's M-series. [0] The 5090 certainly wins most benchmarks but that doesn't mean the DGX Spark is not useful.
0. https://float16.cloud/en-en/ai-benchmark/dgx-spark-vs-rtx-50...