Its hard to say, simply because it's not at all a popular word anymore. The meaning indicates negativity, in the same way "half-truth" generally does, but I've never encountered it in general speech or common use. I had to look it up to make sure I knew what it meant!
Apparently an essay by Francis Bacon, "Of Simulation and Dissimulation", appears to be the most "popular" piece of literature that calls attention to the words and their meaning. The most common reference I could find after that was a reference to ancient Rome! ...and actually, upon further inspection, it turns out that it was indeed the essay of Bacon himself that contained the reference.
So indeed, its connotation will mostly be related to the connotation of its more popular related words and synonyms, which appear to be "half-truth", "hypocrisy", "dishonest", concealment, and so on. It will tend to be negative, though just as "subterfuge" can be meant as a positive thing, as in spycraft, your usage will more strongly influence its conveyed meaning far more than connotation of the word.
But to be fair, most people just will have no idea what you are saying unless its very clear from context, because its not at all a commonly known word.
The verb forms are normally used negatively...
"What are you two nattering [on] about?"
...is more akin to wittering, prattling than chatting, talking.
The adjectival form is almost always negative...
"In the United States today, we have more than our share of the nattering nabobs of negativism"
...as is the gerund noun form...
"By that time their constant nattering had got to me and I ordered a taxi home to enjoy a little peace"
Only the "uninflected" noun form is usually positive...
"I just love a good natter" (Google Books claims over 4000 instances of "a good natter")
So since OP's two examples are both of that type, he needn't worry about negative connotations.
Best Answer
No. Effusive is not negative, it has a positive connotation to it. 'He was very effusive in his praise of the features.' is fine.
effusive
adjective demonstrative, enthusiastic, lavish, extravagant, overflowing, gushing, exuberant, expansive, ebullient, free-flowing, unrestrained, talkative, fulsome, profuse, unreserved
He was effusive in his praise of the General.
Also, This ngram for effusive in his praise and effusive praise shows the expressions are on well-trodden territory: