I'll chime in since I have progressed from Erasmian to an attempt to pronounce Attic Greek (in the 70's and early 80's) when some vowel pronunciations were not totally worked out, to using Buthian (Randall Buth's 'Restored Koine' or 'Imperial Koine' Pronunciation). I discovered in the early 80's that the wide-spread scholastic pronunciation did not mimic actual Attic nor Koine (200 B.C. - 200 A.D.) pronunciation. So during the 1980's I tried to change my pronunciation to mimic the historical pronunciation of the literature I was reading. The diphthong ευ was one of the things I struggled with. After reading Gignac and other articles, I had decided it should be prounounced [ε-o] (rightly or wrongly). I was an aspiring student and tried to read out loud and smoothy. Those were my goals. I did not listen to a lot of audio back then - there was none. I also did not have any exposure to Modern Greek or Greek Orthodox pronunciation.
In the late 1970's, The book Vox Graeca, (3rd ed., Cambridge, 1978) by Sidney Allen came out. Stephen Daitz wrote The Pronunciation and Reading of Ancient Greek: A Practical Guide, (2nd rev. ed., Jeffrey Norton, 1984). Randall Buth has been publishing articles on the pronunciation of Greek in the 1st century A.D. since the mid 1990's, and bases a lot of his work on Gignac's grammar of the Greek papyri.
When I went to the U of M (Minnesota), after having a minor in Greek from a Bible College where everything was in Erasmian, I had a prof who made us read Homer and use meter to read it out-loud. The pronunciation was still Erasmian, but of a sudden, morae, cadence, and prosody became important. That same emphasis on pronunciation was not used in my other classes reading Euripides, Aristophanes, and Herodotus. That emphasis on morae, cadence, and prosody made me rethink how I was pronouncing my Greek. So i started to change my pronunciation. At that time, I did not come up against the ERASMIANS. A lot of Greek students were in the dark at that time.
I took a hiatus from Greek for about 20 years from 1985 to 2005. When I got back into Greek in the mid 2000's, much had changed on all fronts. One of the big changes was the clarity on original pronunciation schemes. No speaker of Greek (native or non-native) ever pronounced ο (omicron) as a short English 'o' as in 'bot', 'cot', etc., which every other language pronounces as [a]. Yet that is what Erasmian and many modern colleges and universities taught. Original pronunciation schemes are now making headway into Greek pedagogy. But on the whole, Erasmian pronunciation is still the most common pronunciation used by traditions GT (Grammar-Translation) taught students of Greek.
The 'standard pronunciation' taught in schools imposes a very odd conundrum on a student who wants to use the communicative approach. Most of those aspiring learners, want to 'lock into' a set pronunciation, so they can consistently hear and speak any given vowel, diphthong, or consonant/consonant cluster with the same consistent identical pronunciation. They would also like the pronunciation they 'lock into' to be used by other learners who are using the same pronunciation. There are very few number of audio books available for students of the communicative ancient Greek.
For the most part, Erasmian is rejected by most of communicative aspiring students and teachers. But the playing field has many players; Christophe Rico used a mixed Erasmian/Modern pronunciation; If a person would listen to Stephen Daitz' pronunciation of Restored Attic, they would listen to what they perceive as a very odd tonal accent and long/short vowel draw/drag presentation of recorded Greek passages; Randall Buth's Koine pronunciation is much closer to modern Greek pronunciation, with the exception of several vowels and diphthongs; Some modern Koine teachers recommend using modern pronunciation (They struggle with pronouncing ὑμεῖς and ἡμεῖς identically ( Modern Greek now uses ἐσεῖς and ἐμεῖς to deal with the identical pronunciation of υ and η).
The change from classical Attic to Koine of pronunciation happened gradually from about 350-150 B.C. (those are my own dates). But by the 1st century A.D., the Attic historical pronunciation was gone, along with the tonal accent, and the new Imperial pronunciation was used by most in the Roman Empire and Hellenistic East. So for communicative students of Koine, there should be no question what pronunciation they want to use. Students of 5th century Attic need to use Restored Attic; Students of the Hellenistic Koine need to use 'Restored Koine.'; by that time stress accent had mostly replaced tonal accent and the differentiation between long and short vowels was leveled (ο = ω; ᾱ = α; ῑ = ι). But the main caveat of any pronunciation scheme is that the ACCENTS NEED TO BE PRONOUNCED AS THEY STAND (excepting instances of emotion and prosody, where an accent is changed from the standard preset 'programatically' written accents.
I've been teaching students 'Restored Koine' pronunciation the last three years . I think the student really needs consistency for about 5 years. But second and third year students get thrown into classes which may use Erasmian pronunciation (vs the Restored Koine pronunciation that is used by most in the communicative approach). I tell them 'Some people talk like they are from Phrygia; I talk like I'm from Alexandria. But, Greek is Greek, regardless of the pronunciation and where you come from."
I've also been in a class lately as a co-teacher in an environment where everyone else only knows Erasmian pronunciation. It is hard to talk about grammar and use the Restored Koine pronunciation [i] for ει, which is not standard Erasmian which pronounces the diphthong ει as [ei]. This difference in pronunciation is especially problematic for students who use very little audio (other than their own reading of a Greek text in Erasmian) when discussing a Greek text. I've found myself as of late trying to 'read the text in Restored Koine' but talk about word forms in 'sounds' they understand. This is a hard and not easily reconciled conflict of pronunciation. It presents a real conundrum for teachers who want to change pronunciation, but who teach classes which have many students who have only used Erasmian pronunciation.
The best suggestions I have are
---(1) Always accent words correctly. THIS IS MORE IMPORTANT THAN ANY OTHER ISSUE.
---(2) Be consistent in your pronunciation
---(3) Listen to different prophora (pronunciations), but try to train yourself to be consistent on only one specific pronunciation.
---(4) Choose the right pronunciation: Restored Koine; Restored Attic; NOT a spell-talk ERASMIAN never spoken prophora.
Hopefully, those of us who are students of Randall Buth's pronunciation scheme will provide a plethora of Greek audio which communicative students can listen to and train themselves.