Also, F/Z has several things that are just plain inconsistencies with FSN, so you really aren't going to get entirely spoiled.
That's to be expected of any prequel/sequel. Or even the primary work itself.
Just out of the top of my head:
[spoiler]
Archer at full strenght gets made into mincemeat by berseker on route 1. When berseker gets pumped out to eleven in route 3 by corrupted grail power, he's hardly a speed bump against brain-damaged Shirou.
First route the priest is some kind of dark magician that ends up defeated in melee. Last route priest is some crazy knive-fighter that can take on servants head on in martial combat.
Why doesn't Shirou just uses Rule Breaker to rescue corrupted Saber in route 3?
Saber, King of knights, master swordswoman with supernatural battle instincts, get defeated in a pure blade fight by caster 2 out of 3 times. Caster that can't even defend against basic martial arts skills.
Gilgamesh walks around with his pimped armor in route 1 that can block heavy magic and even direct blows from non-ultimate excalibur. It is never used again in the other routes despite suposedly being instant-summon.
[/spoiler]
Prisma Ilya 2wei 17
[spoiler]Prana running out. Refill please.[/spoiler]
[spoiler]
One can only wonder why Rin never mentioned they could refill Saber that way back on Fate/Stay night.
[/spoiler]
[spoiler]Shirou is probably too terrible at magic
Though given Ilya's reaction I'm guessing the 'more efficient' transfer methods still work even in the more comedic universe of Prisma Ilya[/spoiler]
[spoiler]
Sure, but I would expect lesbian-Rin to don't pass on the oportunity to kiss Saber on the heat of battle with the excuse of recharging her prana.
[/spoiler]