The strength of the jetstream is not an issue from a point of view of aircraft limitations. However, turbulence associated with the jetstream is a concern for the pilots. Very rapid changes in the speed and direction of wind can create very turbulent and unpleasant flying conditions, which in the most extreme cases can also be very dangerous, and naturally pilots will choose to avoid flying in those conditions regardless of any benefits in terms of time and fuel burn. This turbulence can occur when aircraft fly into or out of the stronger winds of the jetstream, or even when in the jetstream itself. Strangely, the actual strength of the jetstream does not necessarily correspond to the severity of any turbulence.
The type of weather forecasts which pilots receive before a flight are usually quite good at predicting the likelihood and the severity of any turbulence, but are not infallible. In the cruise, pilots rely heavily on reports from other aircraft in the same operating area to monitor any turbulence and plan their routing and altitude accordingly. Operations across the ocean have additional complications however as communication with Air Traffic Control units is very difficult and as such, pilots have limited options for altering their planned flight details.