Guys, with what am saying next am not kidding or being ironic :
My guess is that the mechanic is now working as intended and was bugged earlier making those first 3 robots a bit too easy.
It just was fixed. This is what I believe, I may be wrong though, but I wouldn't be surprised it was the case.
The only mistake is that the fix hasn't been added to the patchnotes.
Now on the difficulty : it has more to do with being used to the old easy way and not adapting than really being difficult in the first place.
As a matter of fact you DO NOT NEED to interrupt all those casts, just enough so it doesn't reach a certain amount : I think it's 10. If you miss a few interrupts it's not a big deal.
You only need 2 players each on 2 droids and 4 on the non tanked one.
Of course what that does is make the raid a little disorganized as to where everybody is during the walker shots and purple dot areas and makes a team that is used to the old way a little off the comfort zone, that's all, nothing really serious.
I'm sorry, but if the robots are casting just as often on 8 man as on 16 man, then there's an issue. You have twice as many people to interrupt in 16 man and can easily cover off all the interrupts. Reducing the cast timer from 1.5 seconds to 1 second is also BS, as it gives players with any lag at all virtually no chance of interrupting. This change also drastically favours groups with primarily instant cast attacks and short interrupt cooldowns, as they will lose the least amount of damage done, and more easily keep up with interrupts. In an op composed primarily of people with 12 seconds cooldowns on their interrupt, this is a problem.
It all adds up to an extremely poor thought process by Bioware. The fact that this change was not announced, but rather showed up out of nowhere, is but another indication that Bioware has made a mistake. There are a number of other bugs that were introduced or re-introduced with 1.4, and it makes no sense whatsoever to post "oh, you can still do it" when there's no reason for the change in the first place.
I suspect that this was intended to be a part of NiM EC mechanics, and Bioware screwed up by making it part of HM mechanics. We are posting here so that Bioware will realize that there is an issue and do something to either explain their actions or fix it. It's not so that we can debate whether or not the content is still doable.