They are very precise as long as the input is correct. However they do suffer a lot from input derived errors. If you have due to pressure change within the time between ATIS-cycles an input error of just one hPa it might make landing on a marginal runway impossible if you follow the GP for too long. Which is the reason why Boeing changed our SOP to only continue visually below any MDA/DA on IAN approaches, the GP is not longer deemed safe below that.
Of course any GP is better than no GP, which is exactly why IAN is such a great tool since it offers a GP for every kind of NP approach, but it remains still a non precision tool with all its limitations. Non-Baro non-ILS precision approaches are something completely different and of course something we hope to see a lot more in the years coming. GLS to CAT I limits is allready operational and we are currently in the certification process to CAT IIIa limits, which makes sense since boeing offers GLS as standard equipment on current aircraft.