It was a success for me with the 3.2.9 to 3.3 automatic update on a test forum but it was vital that I deactivated the following extension otherwise it would crash:
pico/jbreu's Reputation System
And then those extensions would show an error in the ACP:
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That was it, the rest went flawlessly and was working on my local 7.4.1 php version.
I reported it in the support forum of those extensions.
Edit: I managed to fix the above with the help of the members of the forum, I now have a fully functioning phpbb 3.3 live forum with all its extensions