If I recall, the last thread about the Oxford to MK section of EWR got locked precisely because it was devolving into an argument about DOO/RMT.
I had a long online discussion about DOO with various union members against the increase in DOO operation.
They cited safety as being the critical reason why DOO should not be extended, examples cited were driver illness, incidents such as collisions and anti-social behaviour.
I couldn't find any independent evidence online that DOO routes were any less safe, so their safety concerns seem to focus on safety 'incidents' that don't happen very often, e.g. accidents or events that one might say are more 'customer service' related.
Personally, it seems to make sense to have a second person on board, as long as they are visible and useful, to make train travel more appealing and improve overall customer service.
As is usual, the 'customer gets caught between a rock, the unions, and the DfT, a hard place.