Like almost everything, there's tradeoffs. Transcoding to a really high-end single-file codec like Cineon or Cineform would mean that say the file size could be somewhat larger on disc ... more data to read ... however, LESS work by the other subsystems to get those io oo ii oo oo io io io oi bits changed into "video" on the monitor. There are several articles and tutorials out there on this that give really useful information. And there are a LOT of people that put something out because they personally think this or that codec is THE proper way a professional should work, and any other codec is nearly obscene in use. Right, whatever. They're all tools.
As shooternz showed ... properly shot AVCHD can be green-screened beautifully, and as there are a LOT of the "pro" cameras in use shooting that wrapper, there are a lot of people that simply process in that. There are others that automatically transcode everything to ProRes 4444 before editing because they feel that gives them the best, safest "space" for their pixels to live. Pick any major codec/wrapper, it will have adherents.
I've tried just a bit of greening just to play with it ... and got rather intriguing results. It reminded me of starting out trying to do good still portraits on high-key sweep backgrounds without getting "haze" or scatter from too-much-light coming back at the lens while getting the right exposure on the subject and keeping the entire high-key area of the sweep the same white ... it's rather daunting at first. After we mastered it, we could do it at will ... first time every time. Green-screening is a similar thing. Perhaps on steroids, but similar. HOW it's shot is incredibly important to being able to post-process both cleanly and quickly. I've seen quite a few comments about this here and elsewhere ... and though there are clean-up & masking techniques one does need to master, clearly ... one needs clean footage to be able to quickly get clean post work. So many people have noted that after spending so much time working on their masking & color-correction of bits here & there they also got their shooting skills up. And found that suddenly they didn't need anywhere near the clean-up skills or time spent.
So I'd think off-hand that the AVCHD isn't the most pressing problem here ... that a bit of experience in the absolute tight control of exposure and lighting in the shooting setup may well take care of much of the post problems. And then you can concentrate on those skills you do need ... the fine techniques of this particular process ... to get your work completely polished.
And listening to shooternz on this isn't a bad thing at all ... his techniques are tight & fast. I've asked him for a number of things and have adopted every one of them.
Neil