One problem related to brake antilock wheel speed sensor has been reported for the 1998 Volvo V70. The most recently reported issues are listed below.
I bought this car used with less than 35k miles in Feb 2002. Problem was a frozen right front brake caliper and a faulty right rear wheel speed sensor which engages the abs unnecessarily/randomly. This did not happen for about a year after owning the car. I have been told by my mechanic that the rear wheel speed sensor failure appears to be quite common because he has seen it in many of the same cars. The part that is used to replace it is also a different part number which leads me to believe that Volvo is most likely aware of this problem as well but have never issued a recall. There were many other problems with the car such as barometric pressure sensor failure, upper motor mount requiring replacing, ac sensor failure, wheel bearing, etc. All within the period of 1 year but none of these things affected the safety/braking of the car so I do not consider that truly dangerous. Those issues involve aggravation and money, not safety.