Prisma Phantasm is another entry in the growing group of Nasuverse parodies. As usual with those, it's full of meta humor and references, so it's recommended to have extensive knowledge of source material. In fact, even watching all of the *anime* released so far won't be enough to catch everything. Althrough Prisma Phantasm isn't as packed as it's predecessor Carnival Phantasm. Now, as you might have noticed, some viewers have been flabbergasted that this isn't in fact focused on the Illya gang and the usual loli fanservice as the previous OVAs. So, if you want fanservice in the form of visual candy I suggest that you'llbe better off watching those previous OVAs. Because while this might be a spin-off of the Illya universe, it focuses on the universe and various minor characters while leaving the usual protagonists as a side characters. In fact, most of the focus of this OVA is on a "redemption" arc of a certain villain.

So, once you put aside your anger of not getting more lesbian loli fanservice (which is of course a completely understandable outrage), you'll get a perfectly fine comedical OVA with the usual antics of out of characters what-if scenarios. And really, isn't it in the spirit of Fate parodies to focus on the more obscure parts of the universe? I, for one, don't see any problem with that. And so shouldn't you.