Well, I've played Scrapper a bit. Engineer was my main before my Necromancer, so I guess I kind of know what to expect from them. Most WvW Scrappers that I've seen are running with Alchemy and HGH w/Elixir Gun. If you can pressure them early and fast as a Condi Reaper, a lot of Scrappers will blow through their cleanses pretty fast. However, the more elixirs they have on utility and (possibly even their heal), the more of a pain they'll be for the Necro since that gives them more condi clear and boons and their elixirs will generally be faster than your corrupt CD's most of the time.

The other roaming Scrapper build that I've seen (albeit rarely and is much harder to deal with) is if they're running with Inventions rather than Alchemy and they're running with Marauders or Zerks. They'll usually have gyros (which are also becoming more popular) which generate lightning fields for them. Hammer 2 will get utilized here if not near constantly to keep you stunned, alongside with self-destructing the gyros themselves to stun as well. If I'm roaming with the GS Power Reaper, these guys aren't as much of a problem since most of my time is spent in Shroud and you can just shred them pretty quickly. The common roaming Condi Reaper builds should lose this matchup though. They can cleanse conditions pretty quickly and easily and pressure you to a point where you can't do much. If they're running Firearms instead of Explosives, you have somewhat of an advantage because you CAN give their conditions right back but they'll usually CC you to a point where you can't. They'll outrun and outsustain you too.

It's kind of sad that I don't see many WvW Scrappers roaming as much as I used to. They're still very solid. Hell, I'd argue that they can counter most of the roaming meta right now in my opinion. They also have enough mobility to avoid zergs, they have stealth if they need it, and they can 1vX pretty well in my experience too. The only reason I don't play my Scrapper as much is because I just don't like playing piano and I prefer a more calmer playing experience as a Necro. Plus I love life steal and conditions.