Hi, Ádám!
Certainly you should not forget the Ethiopic Synaxarium, translated by Wallis Budge (1928), and also Bayan's Le Synaxaire arménien de Ter Israël (I-XII, 1910-1930 - I have it in one file, just like Basset's, if you need them).
What's the name of the saint you're interested in?
Coptic synaxaria are tons of fun, mainly due to Arabic deformation of names, both place and personal. One of the more hilarious occurences is that, on Epip 10, St Theodore of Kyrene becomes suddenly two saints: T. if Pentapolis and T. of "Corinth". (On Paremhat 6 there is also a "Corinth on Cyprus", which is actually Keryneia.) Or, on Paoni 10, a minor character in the vita called Eudemon (a man) unexpectedly gets the same name as the main character of the vita, Dabamon (a woman), which brings a lot of confusion into this entry.
There are also inexplicable merges of saints (e.g. Roman popes Celestine and Liberius, Epip 3). Or innumerable variants of a saint's name (check e.g. St. Piroou on Epip 2 – 8 variants). Or, how to recognize Apollonius in "Balanah" (Epip 8), or Febronia in "Afrunya/Kafronya" (Epip 1), or Thalassion in "Latsun" (Paoni 17), or Calliopius in "Haboulyous/Hanulius" (Paremhat 4)? Or Tabennisi/Tabenna in "Donasa/Dounaseh/Davanise"?!
If we may assume that e.g. "Patriarch Theophanes of Antioch" (Paoni 25) was indeed some non-Chalcedonian prelate, unattested in any other document but the History of the Coptic Patriarchs, who the hack could be "Acacius, bishop of Jerusalem"?!
There are also some saints whose vitae resemble very much some European hagiographies; for some of them we can say that motifs coincide so much that we can recognize their identity, but with the others it's hard to say: e.g. is the Syriac/Coptic "Hilarius/Helladius/Aladyus" the same as St Basil of Ancyra, or is "Hilaria (a daughter of Zeno)" actually Apollinaria (a daughter of Anthemius)? Could "Philo (Filou), l'évêque de Perse (Faris)" be Abramius/Abraham of Arabela (commemorated on the same day - Meshir 10/February 4)?
However, if you succeed in cracking the names, this sometimes leads to beautiful discoveries of historical facts that are preserved only in these messy documents. For instance, I was amazed that the little-known council of Amastris in Pontus is commenorated as a feast (Paremhat 4) of the council of "Bani-Omar"!
The tendency to misrepresent the functions of the officials is present not only in the very old vitae, but also in those from the 18-20th cc., which could be easily checked and corrected.
The further misconstructions and misspellings continue into our days; consider, for instance, the 1968 transfer of a tiny particle of St Mark's relics (that had previously been kept in the treasury of Venitian cathedral, outside the altar!) that has been recently introduced in the Coptic calendar and commemorated on 4 days (Paoni 15, 17-19) as the "Return of St Mark's Relics" (St Mark's relic of the same dimensions was given by the Vatican to the Greek Patriarchate of Alexandria in 1969 without any pomp). You can find in Coptic documents that the document of the relic's authenticity was signed by "Cardinal Porfeer, the deputy of Vatican city", in reality: Petrus Canisius Van Lierde, a titular bishop of Porphyre[on] (Jieh), RCC Guardian of the Relics, Vicar General of the Vatican City.
Also, St Abraham of Faiyum (d. 1914) was born in "Gilda", which is... (Delga?) There are loads of publications about him, but no one cares to investigate this.
One should also beware of some mistakes of the editors. E.g. Basset (Meshir 8) misquotes Zoeg (Catalogus, 1810) and identifies Astir with Pi-Shati(p) (Shutb/Shotep, near Asyut), thus mixing two different saints.
Misspellings and misidentifications are extremely frequent in the American/British edition of the Coptic Synaxarium (1995/2006).
Unfortunately, the Coptic Encyclopedia did very little to clarify the topography and onomastics of the vitae/synaxaria. The Copts somehow don't seem to care at all!
I believe I've deciphered most of these names (some with the help of the members of the NASCAS mailing list - special thanks to prof. D. Morozov!), so we can keep in touch.
All the best,
Alen