I just made a TC footnote on the variant in John 15:8 regarding γένησθε (aorist subjunctive) in the NA28, versus γενήσεσθε (future indicative) in TR RP. The UBS5 and NA28 footnotes cite ALL the Old Latin manuscripts as being in support of the γένησθε (aorist subjunctive) reading. This is true insofar as the subjunctive versus indicative mood question. But the Latin verbs do not agree with the Greek verbs exactly. They read in fact: sitis (pres subj) “be” it-a,d,e,q,r¹ ‖ efficiamini (pres pass subj) “be made, be proven” it-aur,b,ff² ‖ possitis fieri “be able to become” it-f. Certainly, none of the Latin manuscripts have a future tense verb.
A limitation of Coptic to render Greek is shown in the change from the UBS3 footnotes to now in the UBS5 footnotes. The UBS3 footnote cites cop-sa,bo,ach2 in support of γένησθε (aorist subjunctive) of the NA28, but now in the UBS5 and NA28, the Coptic is not cited for any reading. If you were to read the English translations of the Sahidic and Bohairic Coptic texts, you might think that they do support the NA28 reading, because they read in English, “and become to me disciples” (Sahidic) and, “and that ye be to me disciples” (Bohairic).
However, J. Martin Plumley writes in Metzger’s book “The Early Versions of the New Testament, Their Origin, Transmission and Limitations,” Clarendon Press, Oxford, 2001, p. 149, the following: “The well-developed system of auxiliary verbs in Coptic, while capable of dealing with the main temporal aspects of the Greek verb, are less able to represent the more subtle distinctions of mood, especially the subjunctive. Thus, where Coptic uses a future tense containing the element -NA-, it is not possible to decide whether the Greek text showed the future indicative or the aorist subjunctive.”
Another example of the limitation of Coptic is when applied to the extremely common variant in the Greek MSS of τε versus δε, or ουτε v. ουδε, because Coptic did not distinguish between t and d, and d was usually but not consistently, defaulted to t.
To read the footnote on John 15:8, you can download the gospel of John here.