We saw Sanji get bullied and tortured as a child for this powerup.
This is my personal thought about their power ups:
Luffy - Comes from powerful lineage. He got a df and it gives him few advantages over others but we see how he train hard to master it alongside his haki. Genes + training.
Zoro - Comes from powerful lineage too. He trained the hardest among Strawhats. Genes + training.
Sanji- His genes is modified. He got bullied coz they thought he was a failure back then. They doesn't realise that all this time, the power is there but it take time to awake.
Now, the power is awaken and Sanji become a cyborg. He is more durable plus he got a raidsuit that make him invisible. He can use his rs without any hard training.
All his power ups got no drawbacks unlike Luffy and Zoro. So, I guess that's why some of us think he had it easier. Maybe Oda would elaborate and show more about his training after this.
Even tho I doesn't like his rs and his power ups (I prefer a normal Sanji) but I'm glad to see he level up.