Durax is by far the hardest hero to use. His simple skills make him a very 1-dimensional hero at first, until he clones himself. Then, you can expect to do a LOT of micromanagement, as they are all frail but capable of intense damage.
My one gripe with him is that I can never tell which clone is which. I get that that is the point of a clone, but the one that’s about to die because the 75 seconds have completed is indistinguishable from the freshly-spawned one, and when it may be okay to let the older one die, I might accidentally let the newer one die, and the old one will despawn as well, leaving no clones left.
Still, his hero spell is really cool. I find it much more useful to use on a single target than multiple enemies in a group. It just feels more potent to take out one enemy than to marginally harm a couple.