Maybe not illegal as such but certainly going against the rules. They got rid of concessions for children of all ages and for seniors. This was contrary to Premier League rules and (eventually) the Premier League recognised this and insisted they must be reinstated. Unfortunately by the time this had been formally recognised City had been relegated to the Championship where concessions were only advised rather than compulsory. I think the FA would also regard the request to change the name of the club (without consulting the fans) as dodgy. The decision to evict sports clubs including disabled athletes from the Arena and the way it was handled was widely regarded as dodgy. The closure of the gate to the East (thus making the walkway redundant) was dodgy. To suggest it was done to prevent a terrorist threat (rather than to annoy their friends at Hull City Council) is dodgy.
How do they compare with other owners? Harold Needler was 'generally' well respected and very popular and the rest obviously less so but to varying degrees. Some were no doubt more honest than others. Illegal activities are alleged to have occurred beyond those that were reported. Bartlett & Duffen were not so unpopular at the time but became so when the financial situation became apparent. Duffen was certainly more PR aware than most. Most of the unpopular owners owned the club at a time when Hull City was in the bottom division and / or had not enjoyed any recent success. As a consequence less fans were affected, the club appeared to be relatively small and insignificant and to an outsider it probably seemed there was little there to actually damage.
To me (and many others) the Allams' unpopularity is at a different level. The club had enjoyed success and had further success whilst they were still in charge. Rather than build on that success and at least leave a legacy their attention seemed to focus on falling out with everyone from the Hull City Council, The FA, various managers and head coaches, Hull Daily Mail, Radio Humberside and even through to their very own fans. That is unforgivable!