This is just from my experience and intuition, so take it for what it is.
To me, the usual pronunciation of this word definitely has "r": I feel like I normally say /kʌmftərbəl/ (or /kʌmftr̩bl̩/ or /kʌmftɚbl̩/), with elision and metathesis.
The spelling pronunciation /kʌmfərtəbəl/ would also be possible, and I think I might have heard it occasionally.
Neither "comf-ta-ble" nor "com-fa-table" seem to me to correspond to my own pronunciation, and I haven't noticed other people using these pronunciations either. I would be surprised if I did. But apparently, Dan Bron has heard "comftable", so I may be wrong about this.
The reason "comfatable" seems odd to me is that in general, syllable-final "r"s are pronounced in American English. There are regional varieties of American English that drop or may drop syllable-final "r"s, such as certain Southern and New York varieties, so that may cause this pronunciation to show up for some speakers. As curiousdannii pointed out in a comment, it's very natural for /kʌmfətəbəl/ to develop to /kʌmftəbəl/ by schwa elision, so I would expect the second of these two pronunciations to show up in the same places as the first.
Rhotic speakers of English can sometimes drop "r" (or drop r-coloring from vowels) in some words, but usually only when there is another "r" in the word (for example, it may be lost in the first syllable of "berserk" or "surprise"). Since "comfortable" only has the one "r", it seems unlikely to me a priori that a speaker of a fully rhotic accent would drop it. But I have not carried out a survey about this.
There are certainly people who would proscribe pronunciations with elision and metathesis, such as "comfterble" and "envierment", but I don't think they're perceived in general as all that objectionable. I think the people who complain about them would be people who tend to object in general to pronunciations perceived as "sloppy" or "slovenly".
The main "shibboleth" I am aware of with regard to rhotics in American English is actually a case of rhotic dissimilation, the pronunciation of "library" as "libary/liberry". Some people get very worked up about this, or read a huge amount into it about the speaker's intelligence or education.