I think the Scout was the most underpowered, and the update appear to reflect that. Sure, the Scout was fast and useful, but only in niche situations. So they added new mechanics to make the scout be interesting and utilitarian, without being more deadly. Things like Knockback provided them with ways around people, makeing the Scout more survivable without being more deadly. Bonk gave them a way to deal with sentries that wasn't foolproof, but made them not be stupid when they reached a sentry in a choke point. And the Sandman provided a way to distract and disrupt, and generally support the team indirectly. Also, it gave them the special niche of being able to balance out Ubered enemies, which makes the stalemates-broken-by-ubered-enemies situation much more interesting. I think that adding so many new mechanics was bound to lead to players finding ways to break these mechanics, so it was a dangerous proposition: When I was playing after initial release, there was definitely some exploitation of the Sandman. But Valve will balance it in time, and may have already. After all, they can track stats, and adjust accordingly.