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OMPL planners under differential constraints are not optimal. I, together with OMPL developers, started an effort to support optimal planners with differential constraints but this is requires a huge effort (to do it in a hacky way is not that much, I did it in the past but I think I deleted the branch :( ) and also gives some problems that it is, IMO, not really clear how to deal with them (such as system simulation and 2 point boundary value solvers accuracy). Actually, this is an state of the art problem, not OMPL problem.

The consequence of using non-optimizing planners is that the plans are usually bad. For instance, for a car-like robot it will do lots of maneuvers.

However, the question refer to approximate results. If you mean OMPL approximate solution (there is a path but it does not reach the goal region), then most likely you are doing something wrong: bad parameters, wrong problem formulation, etc. For me it has worked in the past.

Also it is mention that the trajectories are "too much inaccurate". With respect to what? Most likely the OP refers to the optimality of the path (reason already explained).