Dry cake usually means the layers were overbaked, the batter was placed in an overly deep pan, or too much flour was added. Level flour rather than packing it into a measuring cup, and remove the cake when the center shows a few moist crumbs.
Soggy kataifi comes from under-toasting, mixing the pastry while it is warm, or assembling the cake long before serving. Toast it evenly over medium-low heat, cool it completely, and keep the filling contained rather than adding extra wet soak.
Weak pistachio flavor often results from using only a small amount of pistachio-flavored paste among many other ingredients. Use substantial smooth pistachio butter or pistachio cream, and keep the optional tahini modest so it balances rather than masks the pistachio.
Sliding layers are usually caused by warm cake, warm pastry, loose filling, or a missing frosting border. Cool every component, pipe the containment ring, and chill the filled cake before applying the exterior finish.
For runny frosting, chill the ingredients and mixture, particularly if using whipped cream, and beat only until stable. Overbeating whipped cream can also make it lose structure. If the finished cake is too sweet, choose darker cocoa, semisweet or dark chocolate, and a less heavily sweetened frosting.