The main problem I've had with them is that they can make the cable routing difficult. If you flub the cable routing, your braking performance will definitely be impacted. This is probably less of an issue if you use a fork mounted cable hanger. With a stem mounted hanger, there's not a lot of room to get the cable going in the right direction.
Once you get the cable routing perfect, I don't think these levers hurt main braking performance, and my experience is that the cross levers themselves offer a bit more leverage than the main levers.