These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Base Breaker: There are a lot of people that dislike Sirius, and a lot of people that will defend him.
Nightmare Fuel: Medea turned her Master into a living wand... which involves putting her in a test-tube, unable to scream or move.
Chapter 37 mentions a Sealing Designate who had tried to make Pokemon. Harmless, right? Then we get to know what happened when they found it:
One intern had made the mistake of trying to pet one of the little monstrosities. A little yellow rat. After she had been electrocuted with enough amperage to literally fry her heart, nerves and brain, no one else had hesitated in crushing every little mistake that met their eyes into a chunky red paste.
OT3: Rin seems to be aiming for this with herself, Sakura, and Shirou.
Though Word of God states it will become a harem, eventually.
What else could you expect? This is Nasu-verse, after all. And Shirou is protagonist. With BAR.