I only started brewing this year, and only did my first grain brew a few weeks ago, but when I was trying to create my first recipe I tried a few different popular online systems, and one stuck out far far better than all the others I tried. Brewfather.
1) It was newbie friendly, as someone making their first ever recipe it was the only one that I didn't get lost in, not requiring me to fill in data I didn't even understand what the word meant, let alone how to find. Maybe that makes it less flexible to more advanced brewers, I don't know, but in brewfather I had the recipe set up and working in minutes, in some of the others I just found myself saying 'what does that even MEAN?? I don't understand!'
2) It was completely up to date with profiles for malts, equipment, water etc. The malt database was a huge deal, in brewfather I could put in 4.2Kg of Weyermann Bohemain Pilsner and boom, all calculated - maybe I need to edit it to change EBC from 3.5 to 4 to match the batch I bought, but that's all. In beersmith I had to select a generic Pilsner malt and then edit it, find the specs of the malt I had, then edit the values to try and match it, not really understanding the specs and struggling with differences in terminology and values between EU and US - I just didn't have confidence in it being right at the end.
3) Simple and effective 1-button toggle from imperial to metric, no muss, no fuss, just change a settings toggle and it all 'just works'. It even changed the recipe to report colour in EBC instead of SRM, nice.