they wouldn't have made it through to last year's final if not for some overlooked mistakes by opponent and this is mostly die minute win though Madrid has a strategy of playing fast pressing games in their last minutes but I think those stuffs that happened last year won't repeat itself especially Bayern, Bayern could have reached the finals if not for the last minute subs . Thus saying Realmadrid won't have difficulty issue in UCL with their current squad doesn't seem right to me.
You can call it luck, fortune or mistakes, it doesn't matter. this is how they've been playing all those years and they have won 15 Champions League playing like that. There is never a big game in Europe where Madrid completely dominates the game. They can look lost the whole game and still win. It's because they know how to win. What you call luck can be said to be their strategy. Something cannot be called luck if it happens year after year.
What they do is simple, make sure you don't create many chances and even if you create, they make sure they defend well so you don't score, then they make sure they score their own chances.
Against Bayern Munich last season, Bayern had the chance to bury the game in the first leg and second leg but they didn't take and they were knocked out.
Same thing against City. Madrid pressed and scored 3 goals in the Etihad Stadium, went to Spain and made sure to avoid defeat and take the game to penalties.