As far as I already know on Toth & Zorn this fight won't be a big challenge. I mean, raising the HP by around 200k for each Boss and use a medpack at the start of the fight is nothing I would consider a big challenge. With my Sentinel I will for sure do ~600k Damage in that 5:20 min fight all alone to one Boss, so if you see that 2 dps are on one boss it won't be even close to stress us out. Why not raise the HP to something really challenging, like 1.1m for each? So you are not allowed to relax some time or fail to lose your stacks?
I really hope the other Bosses will be somewhat a real challenge. Cause if it goes on like "use random stuff, do more dps, heal more" it won't be a challenge. Sorry to say that but I am just afraid this will be another easy Operation that we will finish in no time and then have to wait months again for new content.
As simply raising HP doesn't quite offer much additional complexity. And you can perfectly adjust for this yourself by choosing to e.g. 6 man the raid rather than 8 man it. Problem is, for seasoned raiders there's little challenge in raids here, or in pretty much all MMO's for that matter.
Whereas for newer players to the genre, I've come across plenty of them who are already struggling enough on EC HM, hardly get beyond the second boss in TFB HM and will probably instantly wipe on Z&T EC NMM.