Eikichi Onizuka is a former bike gang leader who graduates from a third rate college he is inspired to be a teacher and aspires to be one. However his methods would be extreme and can put people in danger.
Eikichi Onizuka in the manga has been described by some bitter critics of the manga as an 'out of control maniac' with regards with his methods and behavior which put people including his own students in danger.
Onizuka is arrogant, self righteous and hypocritical and does not practice what he preaches at one point in the manga he even lectures parents on having faith in their own kids yet Onizuka has no real faith in his students at all no matter what their potential is which he does not genuinely believe in, one prime example is demonstrated when he states his students will always be that to him whether they graduate or not. Also he speaks of giving people a chance yet there he does not and his reasons are often shallow at best.
Also he tends to be judgemental and ignorant such as one point in the manga where parents become concerned about their kids (with good reason if you think about it and the kind of letter that went around thanks to Mr. Teshigawara and Uchiyamiyada), but Onizuka thinks he has the right to pass judgement on the concerned parents telling them that if they don't believe in their kids they should blame themselves. Which are utterly arrogant and ignorant statements ,considering he dismisses their concerns for their kids sure some might not be particuarly close to their kids but who was Onizuka to pass judgement on them?
Fact is, Onizuka never really believed in his students its all a facade.
Also one of Onizuka's big hypocries is how he says taking responsibility for your actions is important, yet he never takes responsibility for his own. It didn't matter how much they put people in danger, how it reflects on others, how much harm they can cause etc. These acts alone show the manga incarnation of Eikichi Onizuka as completely irresponsible.
The Manga Onizuka also did not bother to teach his subject matter.
Also Onizuka shows very little real caring to the people who care about him most notably the main GTO cast plus he shows very little real gratitude or respect to people who help him. In fact Onizuka is barely respectful to anyone even his own boss.
Also the manga version of Onizuka has other options when it comes to dealing with his students and others but he never explores them instead he insists the extreme and dangerous is always the best which shows a scant lack of genuine regard for human life.
Onizuka's methods are often extreme and can put others including his students in danger there are times people could have been killed. But Onizuka seems to have little or no real regard and escapes serious repercussions for his actions.
While Onizuka may have done a few noble deeds but the ignoble deeds far outweigh them because in so many cases there was a better way but Onizuka never bothered to try it.
The 2012 Live Action Incarnation of Onizuka was not much better while he was not as extreme as the manga Onizuka he was just as self righteous and hypocritical.
- The anime depiction of Onizuka  was by far a more genuinely heroic character for all his faults and did show a lot more genuine caring and sincerity then his manga depiction. Also he interacted with his students and friends in a much more human way as well.
- There are many crtiics who criticize the manga quite heavily most notably Onizuka and his methods in the manga but also many plot holes, pushed aside characters, unfocused story telling, lack of proper character development and lack of humanity in many characters but also story arcs and added characters which were criticized and unnecessary who contributed to nothing except underdevelop other characters and made the storyline of the manga more convulated. There were many who found the adding of so many characters to the manga very confusing.
- In fact there are many who crticize the manga version of Onizuka as more a villain then a hero or just a plain out of control psychopath.