Opinions are divided on this, so I'm afraid you may get slightly conflicting answers here. Pretty much everyone agrees that Tuning Mods can be merged safely. Tuning Mods do their tuning thing at loadup and they're done (might be oversimplifying this).

For Script Mods, which is the vast majority of those in the NRaas collection plus of course many players have others (and sometimes it's not easy to determine right away which type of mod from other parties we are really looking at unless we dissect it), there are those who insist that it's okay as well. I say otherwise, have seen far too many players report that their Script Mods do not work well when merged, and the failures can be so subtle as to not be obvious. And there are those who have found that putting Script Mods in one merged package with their add-on modules or tuning files, if any, in another is also a recipe for failure.

And, on the other extreme, I'm pretty sure most will agree that Core Mods (that would be ErrorTrap from here) should never be merged.

My general advice is just don't do it, not even for Tuning Mods. It's so much easier to troubleshoot and remove a single possibly misbehaving mod if they are hanging out loose and as soon as any kind of trouble shows up, having the mods merged together adds new variables to the mix that are much more difficult to work through. Plus the possibility to accidentally end up with two versions of the same mod as they continue to be updated and new versions released seems a bit higher than otherwise.

(sits back and waits for those conflicting responses as predicted)

Do not merge Nraas mods because I did it and the Sims 3 suddenly froze.