RE: Attack Power Maximums
Generally, when I set that attack power maximums in my game I set it with effective DCs (that is, as DCs, with advantages prorated in). Besides the HKA example you used, Hand Attack + Str (especially with the way advantages on hand attack work in 5th edition) is another combo to watch. As far as the Absorption example goes, I use 2 simple rules when evaluating a character that absorbs to attacks: As far as the absorption can raise the attack above the DC limit, it must start at that far below the limit, i.e. if the character has a 30 max absorb to EB, campaign limit is 12d6, EB must start at 9d6 and max at 15d6. The second rule is making sure that the character does not have the abiliy to run around at max absorb for significant amounts of time.
Obviously some judgement must be used in handling DC limits, as some advantages are worth more than others, and stacked advantages always require a second look. When we started our 5th edition campaign, since we weren't experienced using 5th (and most of them weren't experienced with champions at all), I decided to do 2 things with character generation: First was that any powers that turned out too powerful/too crappy would be rewritten after the start of play, and second was that after they had played the character 3 times, they could rewrite it completely, keeping the xps they had earned, in case they had not designed what they actually wanted the first time.