For me, it is a combination of two things. One, the fact that my bars are 600 mm (compared to 380 or 400 mm on a comparable road bike) means that my arms are splayed out at a very wide angle, which gives good control, but pushes the inside edge of my shoulder blades too close toghether, so I start getting an aching, buzzing pain in between my shoulder blades after about 12-14 miles of riding. The second issue is that because there is only one position to put my hands my arms are in the same position for the entire ride which contributes to my general feeling of arm fatigue at about the same point. Also, because of the setup of that single position is probably not ideal for me, it is very easy to forget to keep my elbows bent, and so my elbows and shoulders end up killing me from all the road vibration at this point, too. I never notice it on my daily commute, which is only 4 miles at a go, but if I don't end up buying a new road or cx bike, I think I will need to either shorten the straight bars and change stems, or swap out entirely to road bars (which would unfortunately probably end up costing almost as much as the bike itself, and then I still have the road buzz from an aluminum fork to contend with).

If I never rode more than 10 miles a strech, though, none of these things would be an issue for me, and my Alysa would be the bomb-diggity. So, it all really depends on how much you will ride, how long at a strech, person geaomety etc, etc etc as everyone else has already pointed out.

hope that helps.