Full disclosure. We only took them once from LHR airport to Southampton August 2022. To be honest, I don't remember many details of the trip except it was convenient and reasonable priced. It left no emotional scars from the trip that we can remember after 2 years. It got us to point a to point b as advertised.
Last year we took a train from Civitavecchia to Rome with luggage. While we would do it again, the train was completely full, cramped, and traveling with luggage was a chore.
Of the two experiences, we had no concerns using National Express again. It was 19.00 Usd for the both of us one way to London.