Six Reasons to difficult to understand on network

From Networkbiology.net

1. Structural Complexity: the writing diagram could be an intricate tangle

2. Network Evolution: the wiring diagram could change over time.  On the world-wide web, pages and links are created and lost every minute. 

3. Connection Diversity: the links between nodes could have diferent weights, directions and signs. Synapses in the nervous system can be strong or weak, inhibitory or excitatory. 

4. Dynamical Complexity: the node could be nonlinear dynamical systems. In a gene entwork or Joesphson junction array, the state of each node can very in time in complicated ways. 

5. Node Diversity: There could be many different kinds of nodes.  The biochemical network that controls cell division in mammals consists of a bewildering variety of substrates and enzymes

6. Meta-complication: the various complications can influence each other.  For example , the present layout of a power grid depends on how it has grown over the years - a case where network evolution after topology.  When coupled neurons fire together repeatedly, the connnections between them is strengthend; this is the basis of membory and learning.  here nodal synamics affect connection weight. 

Strogatz SH. Exploring complex network,  Nature vol.410, 8 March 2001, 268-276