#5 He has underperformed in the league in recent seasons

Cristiano Ronaldo has won a total of four domestic and European Golden Boots (second highest behind Messi's record of five). However, none has come since 2015 when he scored a massive 48 La Liga goals.
He scored 26, 25 and 35 goals in each of the last three seasons respectively, and while those are decent enough numbers for most strikers, they pale significantly to the figures he posted in the league earlier in his Real Madrid career.
On the flipside, Portugal's record goalscorer has incredibly finished as the Champions League top scorer in each of the last six seasons (jointly with Messi and Neymar) and set numerous records, including scoring in 11 consecutive matches the last term.
His recent lower performance levels in the league would be a detracting factor in Ronaldo's quest to win a Golden Boot in a third different country and could play a huge role in seeing him miss out on a domestic Golden Boot for the fourth consecutive season.