SIMDe implements the exact semantics of a given ISA using either the native intrinsics, or other platform's intrinsics when possible, otherwise autovectorization. This is great when you've already written code using e.g. NEON intrinsics and want it to run on x86 (similar to neon2sse), or the reverse. It's an impressive undertaking to implement thousands of instructions for each platform :)
Highway instead aims for a path that each platform can implement efficiently. For example, ReorderWidenMulAccumulate bridges differences between NEON and x86 which user code need not care about if they just want a dot product, and CountTrue is efficient on both platforms without requiring NEON to go to the extra trouble of matching the exact x86 movmskb semantics.
Also, Highway uses only intrinsics and does not rely on autovectorization (+), which makes for more predictable performance.
+ except in the EMU128 target, which is only used if SIMD/intrinsics are disabled
