Yes. William is right. There are some disadvantages when you use one single model with 500 plants (5000 polies) intead of 500 plants with 10 polies each.

When you use fog and clipping then the entire 500 plants will get fogged or will disappear. That can look very strange.

Another solution is: You let the engine do the clipping and the LOD. Damocles did a good exsample of this. You create for every plant / tree a few LOD models. The last LOD stage is empty. LOD 2 and 3 are getting smaller, less saturated and a bit transparent. Then the blending will look smoother. When the model gets to LOD stage 4 it disappears completely and will not be rendered. That allows to display several thousands or even more of plants and trees without problems.

And the best of it: You need no action or script that would make it slow.