Hihi, I was reading the recent posts re: this debate at work earlier, and wanted to reply back. I regularly fight against a Siegfried and Nightmare main online, and while you do raise some valid points regarding their range and their attack power, it doesn't make them impossible match-ups at all. In fact, I dare say that Ivy generally has an advantage over Siegfried and, depending on your opponent's playstyle, an even match to very slight disadvantage with NM.
I do understand your frustration with Siegfried and Nightmare and the other characters you've mentioned. If played by incredibly patient players, both Siegfried and Nightmare will attempt to keep you away for the majority of the match as they well know, pointed out by Synraii and Paranormal_Oreo, that Ivy's close-range to mid-range game will generally be faster and safer than anything either utilizes. Once up close, Siegfried's stances are either highly punishable on block or easily GI'ed or JG'ed. As for Nightmare, there are wide gaps between his strings [AA/BB come to mind] and he suffers severely from a whiffed attack [one of my favorite punishes is a JG'ed Nightmare 3AA with 1B+K followed by a bK. "You FILTH!" :)]. Don't forget also that 3B on normal hit will connect with 6B8 quite frequently with Nightmare. 3 of those in one round are no bueno for Nightmares.
Ivy, on the other hand, is for the most part, safe [mostly -6 to -10 with sword attacks, none of which either can counter consistently], has incredible punishers [6B8, 4B, 1B BE], and possesses good grab range that can capitalize on these openings. If need be, she can easily regain the advantage and frame trap for days with 4B, 3K, 3A, 6AK, 4A+B, to name a few.
Also, this may have been mentioned in previous threads, but Ivy has a pretty solid guard crush game that supplements her punishment game considering the safety and speed of her pokes and sword attacks. 44B, 22A, and 6B+K [sword version] are -2 on block and inflict great guard damage. B+K is terrific for this, as well as serving as a great spacing tool and a CS setup for overeager opponents, and, as a throwback to Sora's previous post about the usefulness of this move, 44B+K's 2nd unblockable hit is guaranteed once the first hit crushes, which leaves you a 105 damage Ivy Lick combo with no meter involved. Let's not also forget the kick to the nyuts 3[K] which leaves you with +3 on block and is also a great CS buffer.
Yes, this iteration of Ivy is radically different from what I've seen in SC4, but quite honestly, I actually prefer this Ivy over previous incarnations. She is not meant to have insane normal hit damage nor ludicrously long combos. Rather, she has her speed, frame trapping, and versatility throughout all ranges that more than make up for that deficiency. Played correctly, Ivy has the means to negate an opponent's offense and score great damage through her CHs, all of which she can pull out from normal position without the need for stance. Add in consistent JG'ing, and suddenly, that Ivy looks a lot more frightening. :)