The newer BX's (for sure from 50 series on) are true cat 1, based on cat 1 technical specs. Older BX's were "limited Cat 1" because their 3pt did not meet cat 1 specs. However, even the newer BX's do still have physical limitations due to the height of the BX and the 3 pt's arm lengths and spread. It is a small tractor after all...
So, though they do have a Standard Cat 1 hitch, we need to be mindful that there are some cat 1 implements you can run into that will not really fit, due to the "limitations" of the tractor size and/or arm spread. Folks on here have reported cases where certain implements couldn't be used - I remember an example of a post hole digger (too long for the BX), and I'm sure I remember at least one other example that was given.
I've never heard of an alternative that couldn't be found though. That is, there are other cat 1 post hole diggers made for use with SCUTs, etc.