Ugh. Finding plates underwater just like in BW(2) or without a time limit?
Also the fact that Yveltal's revival animation takes too long
There's no time limit this time, but they're hidden (aka no Pokeball to indicate there's an item there) and you pretty much need a picture guide to figure out where you should be clicking around (I tried following Bulbapedia's directions but sometimes I had no idea what they meant). And they're not all in one route, they're spread throughout the various underwater routes (though thankfully none with the strong tides).
I wouldn't be so bad if they let you use Item Finder, infact according to Bulbapedia there's other items hidden underwater. But apparently googly antennas don't work underwater, maybe if they were dowsing rods it would have. *shrugs*
Yveltal/Xerneas revival wouldn't be so bad if they left you check its stats before entering the next cutscene. I'm finding GameFreak is falling into a very bad trend. In XY we had Yveltal/Xerneas and in ORAS we have Latios/Latias & Rayquaza (Kyogre/Groudon & Deoxys as well if you want to catch them immediately, but you can knock them out and catch them later). Let us 1. save before facing the Legendary and 2. given a break to check its stats before continuing.
"But it's cinematic!" If the story is what you're mainly worry about then don't make us capturing a Legendary part of the story! You did it in BW2 when we faced Ghetsis. Either give us the chance to capture the Pokemon we want or don't make it possible during the story.