Nutaku has average games at best (many poorly translated from Japanese DMM). While many games you can start off as free to play, they tend to quickly ask for more and more money as you progress or else you fall behind everybody else and the game's difficulty increases.
The vast majority of games require A LOT of grinding which means A LOT of your time to unlock good content (taking away the pleasure portion). Those who are ahead, which are almost always those who have paid, also get significantly better awards to open the gap between those ahead vs those starting or falling behind (this destroys any sort of competitive feeling of its users)
In a Nutshell, Nutaku is a money drain for average games that will require you to drain significant time and money and fully supports "pay to win". It is defined as "the rich get richer, and the poor get poor-er" with its games.