I don't think the main cast of Shinkenger are stellar actors. Tori Matsuzaka isn't that great, you can tell that he was primarily a model before he got the role, and he's the one that gets most of the screen time. They did improve throughout the series, however, which gave us some really powerful scenes by the end. Jii's actor, Mitsuru Karahashi (Juzou) and Romi Park were excellent for supporting cast. But the off-screen camaraderie of the actors playing the Shinkengers came across on screen and added a lot to their performance. (Just from blog posts, stage show stuff, etc. they seem to be good friends in real life) So they had a really good team dynamic, whereas in Samurai, it's not really there - although the actors play friends/teammates on screen there is something very stilted about their acting
TL;DR: I don't think the Shinkengers' actors were that good. But they got better as the show went on, and their enthusiasm made up for a lot. The Samurai cast don't seem to be trying that hard (with the possible exception of Mike's actor) so their performance seems weak. But, like the Shinkengers they could improve by the end