I am a regular user of LD Lines' service from Dover to Boulogne. It is true to say that the service has been variable during the eighteen months of its operation.
Current problems stem from the use of a new ship on the route, The Norman Bridge. When it was introduced in March there were problems with it not being able to open its bow doors, resulting in lengthy turnaround periods at both Dover and Boulogne. This resulted in a revised (reduced) timetable being introduced. On my last trip, two weeks ago, Norman Bridge had been replaced by Ostend Spirit, which, as Norman Spirit was the previous boat on the route before it was moved to operate the Ramsgate to Ostend service. Apparently, Norman Bridge is now in dry dock for repairs, and nobody seemed sure if or when it might return since it was on a short lease anyway.
My experience has overall been good, but because of the few crossings each way, if anything does go wrong it can create difficulties.
If you want to be sure of your crossing, then I would opt for P&O to Calais. I always tend to avoid Sea France because of their susceptibility to short-notice industrial action.
Hope that helps,
Andrew