Another issue with lots of older processors is that they would slow down clock speed when using SIMD instructions so much that there was effectively no performance gain. You had to be very careful which instructions you could actually use.
AFAIK this was only ever really true for AVX-512 (when touching the actual wider registers). Most others have had a moderate downclock, but still normally worth it.
You're right, it's been a while since I dabbled in SIMD.