There are different challenges to both playstyle. Stability is definitely one of the biggest challenges (stamina was never a huge problem for me, but I get where that comes from), as might be the case with speed, but if you play with the keyboard too, you need to have actual motor coordination with your left hand (or right hand, if you're a lefty), which is something I don't since the left side of my body is borderline retarded
Having said that, I do think playing pure mouse is easier, but that's just because of my playstyle (how I've always played osu! and other games), how (and how often) I use the computer, and most importantly, how well coordinated I've always been with my right hand (and retarded I've always been with my left). It all comes down to personal preference, though, but it's mostly agreed that playing with the keyboard is easier because reasons. A bunch of them.
Having said that, I do think playing pure mouse is easier, but that's just because of my playstyle (how I've always played osu! and other games), how (and how often) I use the computer, and most importantly, how well coordinated I've always been with my right hand (and retarded I've always been with my left). It all comes down to personal preference, though, but it's mostly agreed that playing with the keyboard is easier because reasons. A bunch of them.