I recall when Spirit of Britain arrived from Aker Finnyards, she had a few teething issues. Even during the sea-trials Aker had to do some fine tuning, (Tho I don't recall Spirit of France having any issue).
I agree, its a PR and logistical screw-up, but even highly experienced yards like Aker Finnyards, who built a whole string of vessels (including Ulysses), can have issues.
Likewise, van-der-Giessen de Noord, despite building a vast number of very successful ferries - Konningin Beatrix (1986), Norbank (1993), Norbay (1994), Isle of Innisfree (1995), Isle of Innishmore (1996) etc, were late delivering Mont St Michel to Brittany Ferries.
I am not sure the pricing structure, but I was quoted 307 return for two people, from Dublin Port to Cherbourg on WB Yeats for around mid September. The 150 voucher or the option to sail on an older ship is quite dissapointing. Though to be fair to Irish Ferries, Brittany Ferries are being quite brazen charging around 800 for a return with a car, 2 people, and a cabin, on a "very basic - no frills" Visentini - a vessel comparable to Epsilon and Stena Horizon. Even at the very end of the season Brittany Ferries are pricing 600+ and that's not including restaurant etc. I understand there is more fuel consumption on a sailing to Spain vs one to France, but I don't recall P&O charging anywhere near that to sail on the much better appointed (and thirstier) Pride of Bilbao (Portsmouth to Bilbao).