I think in large part alot of the issues can be seen and dealt with through proper play testing.
The first Arc I created had characters in it with a slow which didn't bother my DM/SR scrapper so I didn't think anything of it. Then I played it with a SG mate who I watch group after group have to run through that junk and thought "that looks like a pain in the rear" and promptly changed the custom character to eliminate the issue.
Also along with varying classes and levels of the characters you use to test your mission...solo versus group size has an impact as well. I found this out that certain combined powersets for mobs you didn't notice solo or one other person but when the groups got bigger and more mobs were stacking certain powers it got much rougher than I planned.
Generally when creating custom groups I try to keep an even level of melee/defender/controller/blaster types at all the levels so no one thing is overloaded i.e. holds...debuffs and such.
Better and more thorough testing all around I think will yield better results but I'm guessing most people don't have an idea of how to properly test what they've created. Not the fault of the individual just a lack of knowledge in that arena. The creation is the fun part and testing can be tedious so I think Arcs get rushed out on that end.
