I got over the fact that IPB4 lacks hooks/mods to begin with. If you were upgrading, you should have been prepared for those mishaps. I for one waited till the essentials were updated to IPB4, then I upgraded from IPB3.4. That being said, it's much easier to do a lot of things that required hard modding, and "recaching" isn't something should be needed all the time to begin with. The general complaints in this thread are mainly that previous mods/hooks are not available, which isn't the fault of IPS rather the devs themselves for not caring. Like anything that upgrades, it's up to the devs to see if they like to upgrade or not, no matter how tedious it is. Yes, the overall syntax for IPS is different and weird, but it isn't something devs are worried about. IPS works to repair the bad habits it made clients learn, and bring IPB to 2015 standards for mobile usage. I think it's great, and each update (even 4.1) is a promising future for 4.x overall.
The database is probably corrupt yes, I have bugs with only my admin account, such as when sending a message, it sends twice! Design bugs only when I connect with the admin account! Many plugin that no longer works as it works very well on a clean install etc .. You want anything else as information?