I'd say that it was quite a handful honestly. With all these new powerful creatures, I felt legendaries didn't have that same,special spark like in previous games when there were only 3 or 4. It didn't seem like an achievement when you did manage to get your hands on one, since there were many more to capture.
I suppose the developers did this, since they wanted to give players a challenge, but it was sort of overkill for me. I preferred if they had a little less so capturing one could become special again :3. Capturing a legendary in the 4th Gen, was just like catching a Shinx or something, as they were rather common.
Also, if I can remember correctly, wasn't Manaphy and Phione introduced in the 4th Gen :3?