sheik loses to ICs because on top of her not dealing with the separation well, she can't contest the space control in the neutral as well as some other characters. Also, dthrow chain grabs sheik, even with only sopo. It did in melee at least, but I must admit I have not seen anyone play with ICs against me.
I dont understand why you would think that she has an even matchup against falco? Its not as bad as fox I mean, but its still slightly disadvantageous for sheik, because of her lack of a proper neutral. Its probably slightly in falcos favor.
I think that she has much more even matchups than she does favorable. Pit, ness, donkey kong, toon link, ROB, g&w, and olimar, are in no way advatantageous for her. They are most likely even. She does well obviously, but theres nothing specifically about sheik that allows her to win those matchups. She doesnt play off any weaknesses like she does to captain falcon.
Samus definitely loses to sheik. Her high priority attacks make samus' poke game slightly obsolete. This was a similar situation in melee.
Sonic vs sheik is terrible for sonic. His attacks have horrendously low priority compared to sheiks, even though they are fast. Sheik can beat sonic with neutral airs and back airs very easily, and pretty much nullify all of his attempts at approacing with spin attacks or aerials.
How does sheik lose to lucas? Lucas is good, and capable of dominating the neutral, but not as effectively as fox can. He is still susceptible to sheik's normal strategy, tricking into unfavorable conditions.
I think the mario matchup might be slighty in marios favor, because of sheik's inability to effectively approach from the air and get passed his fireball spam, but I havent played too many marios.
I feel as if the pit matchup is really strange, and slightly in pits favor, coming from a pit secondary and a sheik main. Pit just beats her in everything, other than her up b being invincible. Its probably slightly in pits favor, but still extremely doable.
Meta knight beats sheik. My training partner plays a very good metaknight, and I have a lot of experience in this matchup. He controls the stage too well for sheik to play favorably, similar to fox. Its still not impossible, but slightly in his favor for sure.
Link is probably in his favor. He can chain grab sheik at low percents using dthrow. He limits sheiks ground options easily with projectile walls and can easily camp sheik, who doesnt have too great of an approach. However, once you are up close, you can pressure him very well, and its very hard for him to get out. Its his grabs that are the real problem. Chain grabs, and at higher percents, dthrow into dair is rough.
I have a lot of experience with sheik, as she's been my main for as long as remember.
And for the record, ICs for sheik is terrible. I secondary fox in melee specifically for ICs and jiggs. If you havent been dthrow chained, you're not playing a very capable ICs. They should be able to somehow grab you. If you're set on doing the matchup though, its not too impossible if you can platform camp them and space them out. As with anyone else, dont let them back onstage once theyre off.