How is Shiki?

She's really bad.

I really like her myself. I've had her since i first started and she has never let me down. Her damage isn't great but she works really nice in an arts stall team thanks to her high HP and amazing short codown heal. However like i said her damage is really bad and her NP doesnt hit for that much, but it,s not bad for farming thanks to the instakill on it.

TLDR: Above average staller, lackluster damage, decent farmer

PS: I've got her at level 90 with max fous and with her first skill at level 10 while the rest are around 4.

She's a great Arts staller, with one of the highest HP thresholds in the game coupled with a massive heal with her third skill. Which means she does pretty below average damage for a five-star. Her gimmick is instant-death, the most inconsistent effect in the game. She's by no means awful but, you already have Nero who has herc's bond CE as a skill taking care of a stall/arts saber and Altera/Salter as a farmer.

Honestly, she's kind-of outclassed by your own saber line-up.

Many people rolling for her because she is Protagonist in another NasuWorld. And she is indeed very beautiful in FGO, even can be compared with Summer Servants

For gameplay, Shiki is not quite special besides of AOE farming. We have lots of AOE saber like you mentioned. Shiki is slightly have better survivability than Salter and OG Artoria; or better damage than Nero OG. However, let us make it clear, since the instance death mechanism is broken in FGO, her performance is inferior as SSR. Also her NP gain is good but not consistency as either Archuria (with same healing skill) or Surfer-Mo (with same Art AOE NP) due to short-lasting 1st skill and no instant-charge.

If you can make Art team for her, she is still good especially when facing mobs. Besides of that, other AOE sabers would be same and even better. Even though, for a stall Art team she cannot provide team buff (like Nero Bride or Archuria's 1st skill) or strong debuff to enemy (like Orion's NP or any decharge debuff)i.