It has nothing to do with difficulty. Actually, the street swing dancer's WC syllabus is much higher in difficulty than a typical ballroom syllabus. For example, most ballroom syllabi save all 8-count figures until full bronze, which is typically somewhere between 6 to 18 months into one's education. The street swingers teach a whip as one of the first 6 to 8 patterns, usually within the first few weeks. By 18 months, most street swing students can not only dance almost ALL 8-count syllabus patterns, but they can usually improvise a whole host of embellishments, variations and syncopations to boot.
Also, the Whip itself has a coaster step in it -- both on the ballroom and the street swing side -- so to say that a coaster is too difficult for a Sugarpush but not for a Whip is skewed. Your personal instructor has no idea why street swingers disapprove of the coaster as the ending to most figures, so he/she has simply assumed it must be due to a lack of ability (or lack of faith in the students' ability).
The truth is, street swingers disapprove of the coaster as the ending to most figures because it represents a complete disregard for lead & follow. The man is not moving back on count 6, so what would cause the lady to come forward? An arbitrary yank of the arm? Let's hope not!
Because the man holds his position on 5&6, so should the lady. If she comes forward, the connection will be lost, at least temporarily, and she will be unable to follow the man's lead. In fact, by stepping forward on 6, she has already disregarded his lead, so that represents a minimum of two following mistakes.
When the lady anchors on 5&6, not only has she followed the man's current lead, but in so doing she has also maintained the weight connection, enabling her to follow subsequent leads. If he decides to do something unexpected (eg continue to anchor for another two beats), she'll be ready.
Stylistically, the anchor also allows for more variety of movement, and is typically slinkier and sexier than a coaster step -- much more in tune with the character of the dance. Coasters are more appropriate where more body flight is needed, for example on counts 3&4 of a Whip.
So why do street swingers condone the coaster on 3&4 of the Whip? Simple: It's what the man is leading the lady to do.
Regards,
Jonathan Atkinson
www.ballroomdancers.com