@Chris You're right comparisons are fun talking points as there are ways of comparing players but factors must come in to play.
As you have compared Scholes, Gerrard and Lampard, it's a fair comparison that you've laid out as there's context and thought behind it. But when people use the whole "Gerrard never won the Premier League" as a way of ruling him out of the debate is where the comparison becomes void because you can't base a players career and ability on team performances. As the saying goes "1 players doesn't make a team but he can make a difference" and that's what Gerrard did. Had it not been for Gerrard, Liverpool wouldn't have achieved what they did. Especially in 2005 against AC Milan. He single handedly dragged Liverpool back from 3-0 down and got them that UCL.
Then, if they decide to go down the route of team performances, take Lampard and Scholes out of their clubs that they had success with and trophies they won would Chelsea and Man United have still gone on to win those things? Quite possibly cause they both had great players.
100% agree with this many pundits like Gary Lineker also mention this when talking Sven. Plus add in the fact that England at that time was divided with the club rivalry that was going on behind closed doors which didn't help matters. Sven never found a way to get Lampard, Scholes, Gerrard and Beckham working as a super midfield that on paper it should have been.
This is another comparison that is subjective to factors. It's a great debate and can go either way but when things get thrown in about Ronaldo at the age of 21 compared to Messi at the age of 21 is incomparable as Messi was at Barca with the likes of Xavi, Iniesta, Ronaldinho etc. While Ronaldo was coming from Sporting to trying to make it in the Premier League with Man United. There's a big difference. But as you said when they were in La Liga the comparison becomes a level playing field then up for debate.
I do get the whole point of comparisons to add to the team but when it comes to transferring players, there's more of an assessment in the comparisons as they compare playing styles for how they'd fit in the team as you said.
There's ways of comparing players and then there's comparisons that need to have factors taken in to account.