Hamilton points out the flaws in reading 'Shiloh' as a person's name:1
Without emending the Hebrew text in any way, one may read this line as "until Shiloh comes." But this reading is strange for several reasons. First, it combines a feminine subject ("Shiloh") with a masculie verb ("comes"). More importantly, what would such an expression mean? As a person, whom would Shiloh represent? Elsewhere in the OT Shiloh is only a place. Why represent an individual by a city, and why represent someone in a message to Judah by a city that falls within the territory of Ephraim?
Several scholars mention the common view that 'Shiloh' is the name of or referent to the messiah, and they even indicate this was 'common' in ancient Judaism, but Lanchester disputes the plausibility and popularity of this intepretation by saying 'it rests on nothing earlier than a fanciful passage in the Talmud'.2
Hamilton and De Hoop3 both point to one possible interpretation that Shiloh is in fact referring to the city, and the passage may be read as 'until he comes to Shiloh', so that the text becomes a prophecy about a Judean ruler (i.e. David or his descendants) consolidating the northern tribes into their kingom. This is criticized, however, because 'the people of Israel never did become monotribal although they were for a while a united kingdom'.
Davidson summarizes another option, followed by the NEB:4
the N.E.B. rendering, so long as tribute is brought to him, involves a redivision of words, but makes good sense in context and continues the thought expressed in the first half of the verse. Judah's power will last so long as others are prepared to recognize it and continue to pay the tribute that vassals ought to pay to their overlord.
The Septuagint supports this reading of the text, having translated the passage into Greek as ἕως ἂν ἔλθῃ τὰ ἀποκείμενα αὐτῷ, 'until the things stored up for him come' (NETS).
The New Oxford Annotated Bible, page 78, gives a footnote saying the 'obscure' passage 'appears to predict rule for Judean royalty until Judah's Davidic descendants achieve universal dominion'. It connects the passage to Numbers 24.17 and Psalms 2 and 110, all pertaining to a highly optimistic perspective of Judah's monarchy, and so concludes the passage 'must be preexilic'.
1 Victor P. Hamilton, The Book of Genesis, Chapters 18-50 (1995), 659.
2 H.C.O. Lanchester, The Books of Genesis 25-50 (2014), 108.
3 Raymond De Hoop, Genesis Forty-nine in Its Literary and Historical Context (1999), 126.
4 Robert Davidson, Genesis 12-50 (1979), 305.