Personally, my opinions on these players aside, I do feel people don't take into account that both Messi and Ronaldo are in "super teams" that dominate their league year after year. Yes they play a huge part in them being super teams, but you do have to account for it. Simply stating goal tallies of them both as some factual indicator of superiority is silly.
I agree with what you are saying. Here is a look at the average number of goals scored per game and average number of points won per game by clubs during Ronaldo, Cristiano Ronaldo, Messi and Maradona's time at each club.
Ronaldo (Brazil)
Barcelona
96/97 2.47 gpg 2.14 points per game
Inter
97/98 1.82 gpg 2.03 points per game
98/99 1.93 gpg 1.35 points per game
Real Madrid
02/03 2.26 gpg 2.05 points per game
03/04 1.89 gpg 1.84 points per game
04/05 1.86 gpg 2.11 points per game
05/06 1.84 gpg 1.84 points per game
Cristiano Ronaldo
Real Madrid
09/10 2.68 gpg 2.53 points per game
10/11 2.68 gpg 2.42 points per game
11/12 3.18 gpg 2.63 points per game
12/13 2.71 gpg 2.23 points per game
13/14 2.73 gpg 2.29 points per game
Messi
Barcelona
08/09 2.76 gpg 2.29 points per game
09/10 2.58 gpg 2.61 points per game
10/11 2.5 gpg 2.53 points per game
11/12 3.0 gpg 2.39 points per game
12/13 3.03 gpg 2.63 points per game
13/14 2.63 gpg 2.29 points per game
Maradona (just looking at the average goals scored by his team as there was only 2 points awarded back then for a win instead of three. This meant teams were more happy to settle for a draw and didn't push on like teams do today to get a goal and the win, making it more difficult for attackers).
Napoli
84/85 1.13 gpg
85/86 1.17 gpg
86/87 1.37 gpg
87/88 1.83 gpg
88/89 1.68 gpg
89/90 1.68 gpg
Overall these stats strongly back up the claim of how the big clubs are more dominant these days and that while it is by no means easy to score goals and pick up victories, it is not as difficult as it used to be. The vast difference in resources these days has seen to that. It also shows how difficult it was to score in Serie A back in the 80s. There as a season Maradona won the top scorer award with 15 goals. Compare that to the ridiculously high tallies Messi and Ronaldo finish with these days. It was a very tough league filled with top defenders and when you take into account that these great defenders were given more leeway than defenders are today in terms of putting in strong tackles, I am sure you could appreciate just how difficult it was for the attackers at the time.