There are a few disadvantages to AI5 that make it undesirable for most people, but basically:
Compared to t50:
the base electrum reward is only slightly higher but games will probably take longer due to higher starting hp.
The larger and generally more varied decks mean much lower chances of winning cards from AI5.
The fact that some of AI5's cards are upgraded and some are not even further lowers the chance of winning them
No rare farms
Difficulty on both can vary significantly but at least with t50, you know which deck types are more popular at any given time so it's somewhat more predictable.
Compared to FG's
Only some cards are upped, not all - again meaning lower chance of winning them(and especially of winning the upped cards)
Slightly lower base electrum reward
Generally easier, but again, more varied opponents making it harder to design decks that can grind them effectively
So basically, if you're looking for rares, t50 is better. If you're looking for upped cards/faster electrum gain, fg's are better as long as you have a deck which can handle them effectively(and it doesn't necessarily take a very high win % against them either for this since even winning one upped card will make up for quite a few losses). Several decks have been designed which are very effective for grinding both t50 and fg's and a deck which can beat AI5 consistantly probably can beat at least some fg's as well or at most might need some minor adjustments to make it work(in fact fractal/pest/vampire was the first deck I used for fg's and was at least decently effective for it)
Combining all of these factors, many people will probably do AI5 once for the quest and then never look at it again aside from occasionally for a change of pace when they're bored. That being said, if you do find you're making more money doing half-bloods than the other levels or just enjoy it more, keep doing so
