Well, the PowerMac G5 talks a big game but isn't actually that great as a computer. We have two Pentium4 machines in the house and I'm pretty sure that they both not only get more use than the G5, but are also more powerful; and that's not to mention the fact that they're still useful because they can run supported operating systems (XP and 8) and modern software with some semblance of competence. The only things that the G5 can do well (local audio/video playback, light web surfing, emulation) can be done just as well on any halfway decent smartphone (grab a video-out cable, a mouse, and a keyboard, and you've basically got a computer that fits in your pocket that is infinitely more useful and somewhat more powerful than the PowerMac G5; a smartphone sucks a lot less power too). That's right, the PowerMac G5 gets its ass whupped by any modern smartphone in terms of performance.
Going back to the Canon camera you were talking about that I use - the G5 falls flat on its face for video editing too. Even though it has a half dozen firewire ports, it doesn't support many (if any) peripherals (the microphone jack isn't actually a microphone jack, so screenrecording is silent too). Even my Canon camera (which is only a few years older than the PowerMac and shouldn't have any problems) simply refuses to work with it. Even if it did work, it's not like there's any good intermediate video editing software either.
As you mentioned flash player not being updated, that's just the way it goes. Even if it were updated, the PowerMac G5 sucks for web browsing in general too (even with the renowned best-of-the-best TenFourFox browser installed) - it's slow, unresponsive and is just a lackluster experience in general because of this computer's complete lack of power [and modern javascript-heavy webpages].
Lastly, being an unsupported processor architecture, you can't install Windows. And while you can run a couple Linux-for-PowerPC based operating systems, none of them work at all out-of-the-box with the useless NVidia video card that the thing comes with (which is completely incompatible with the PowerPC linux drivers). As if to add insult to fatal injury, video card upgrading is a ridiculous game that involves finding a compatible video card (of which there are very few) and using a normal computer to flash its bios before it'll even work in the G5.
Maybe some of this complete lack of performance can be chalked up to the crappy video card that's in there, but I keep in mind that other P4 machine that doesn't have a very hefty GPU either, and it still powers through a lot of stuff that the G5 struggles with. At the end of the day, the PowerMac G5 requires so much hand holding (that is, help of a regular computer) to complete everyday tasks that I just avoid using it. It's currently set up in the basement serving as an emulator computer for NES, SNES, and other retro game systems, and as sad as it is to say, that's all it'll ever be good for.
I might do a video summarizing the thoughts of this comment before I sell the G5, but that's the only plans I really have for it the way it is. Like I mentioned before, its pretty case talks a big game, but it's just not practical for any task that can't be done with a halfway decent smartphone these days.