Since the Giant Robos were introduced in Battle Fever J (1979), the mecha have been a combination of puppets, stop-action animation, some robotics and men in boxy robot suits. Puppets like Magi Dragon for example, using wires. Stop-action like some robots or animals. Robotics like flying jets using radio-control, which I think they used for example for the Triceratops in Zyuranger. And men in suits were used for Tyrannosaurus in Zyuranger and the beasts in Gingaman.
But beginning in 2000 with Timeranger, they started using CGI. GoGoV is the last one that used wires, stop-action and radio-control exclusively. Gingaman had some early CGI when GingaOh was formed. Timeranger used CGI for the flying jets (above). Stop-action, suit acting and radio-control were used for the V-Rex (Q-Rex).
Gaoranger was the one that used more exclusively Computer Generated Imagery for the power animals themselves, megazord formations and megazord finishers. This trend still continues to today. 'Megazord' battles are now usually CGI with suit acting with very little stop-action animation (like with the Origamis). But it is not always CGI, for example with Gao Lion, when he would bite, we would see a close-up for a puppet (scale model). 
Here with Gokaiger, we see scale models, which is still used but limited.
Anyway, the debate is that they should not use CGI or do better CGI. The pictures above are from Tomica Rescue Force, where they use arguably better CGI. Fans are boards have been saying that Toei have to upgrade their software. But the Tomica series has ended and Toei has to do like 50 episodes a year, with almost zero breaks. I do agree that the CGI could be a bit more detailed.
I think the reason the CGI is not up to par and not super detailed or 'realistic' because of costs and time. But I think it can be argued that the CGI of the Megazords now are way better than the graphics used for the Ninja Megazord in the Mighty Morphin movie.
