Harold Wentworth & Stuart Flexner, Dictionary of American Slang (1960) has some interesting commentary on suck and the seemingly allied phrases suck around, suck [someone] in, suck off, and suck up to [someone]. Of that entire group, only one term, suck off, is characterized as "taboo" across the board:
suck off [taboo] 1 To commit cunnilingus or fellatio. --> 2 To curry favor with a superior or influential person. Fig[uratively] and scornfully, to be willing to do anything to curry favor.
Wentworth & Flexner lists suck itself as taboo in the first sense of the word, but draws a fine line between suck and suck off in the second sense:
suck v.i., v.t. 1 [taboo] To perform cunnilingus or, esp., fellatio. --> 2 To curry favor with people in in authority. Although not taboo, prob. from "suck off."
The other three closely related terms are not treated as taboo in any respect:
suck around To hang around a place or person with a view of gaining preference or favors.
suck [someone] in To deceive; esp. to deceive by making false promises.
suck up to [someone] To curry favor with someone by being exceptionally agreeable, or by doing menial jobs for that person. Cf. suck off.
And finally, Wentworth & Flexner offers this entry for egg-sucker:
egg-sucker n. One who seeks advancement through flattery rather than work; a "weasel."
This last term may help explain the non-taboo status of many of the terms in the suck family as of 1960. The notoriety of weasels as egg suckers goes back at least to Shakespeare's day. Indeed, Shakespeare alludes to this reputation twice—in Henry V:
Westmoreland. But there's a saying, very old and true, —
If that you will France win,
Then with Scotland first begin:
For once the eagle England being in prey,
To her unguarded nest the weasel Scot
Comes sneaking, and so sucks her princely eggs;
Playing the mouse in absence of the cat,
To spoil and havoc more than she can eat.
and again in As You Like It (in the voice of the melancholy Jaques):
I can suck melancholy out of a song, as a weasel sucks eggs.
It may be, then, that saying "You suck" to someone has always had the fallback implication "You are a weasel" if the insinuation "You [metaphorically or actually] perform oral sex on people for personal advancement" seems too harsh or dangerous for the speaker to own up to.
To complicate things further, in the 55 years since the first edition of Wentworth & Flexner appeared, the term suck has acquired additional and more generalized meanings. Thus, Robert Chapman & Barbara Kipfer, Dictionary of American Slang, third edition (1995) opens its entry for suck this way:
suck 1 v by 1928 To do fellatio =EAT 2 v (also suck rope, suck eggs) by 1971 To be disgusting or extremely reprehensible; be of wretched quality; =ROT, STINK [examples omitted]
So "You suck" can be used jocularly in the sense of "You stink" or vehemently in the sense of "You are reprehensible" or contemptuously in the sense of "You are willing to provide sexual favors to someone in return for unspecified rewards."
Under the circumstances, it's easy to see why the person telling someone "You suck" and the person being told by someone "You suck" might seriously disagree as to the phrase's meaning.