The Arms are the biggest weak point then the drive shafts. wheels and the diffs (Diff housings really). The arms can be upgraded pretty reasonably but the drive shafts and diff housings are expensive to replace. I also killed the 2 speed on the 3906 several times.

All of these problems are worse if you switch to brushless. I couldn't keep the 3906 running with an HV-Maxx 4400 installed. The 3905 was only a little better.

From what I understand the brushless E-Maxx is better but i'm not spending $600 to find out.