Re: Results speak for themselves.
" although that Renault engine was down on absolute power it was reputed to be more driveable and (I think) fuel-efficient than the Mercedes and Ferrari engines."
False. God you must be old. That was true in 1983-5. But not any time recently.
Do please consult https://www.grandprix247.com/2017/07/02/inside-line-why-is-the-honda-engine-so-bad/ , and read the commentards' remarks.
Supposedly, the Mercedes F1 power unit (2014-) employs a technique 'turbulent jet injection' which, if you're good at it, affords more power (or torque, or economy, or ...) and also reduced emissions. Mercedes had a satisfactory power unit from the start of the current engine rules in 2014, Honda's power unit debuted with this technology in 2015 but the engineers haven't got it right yet, and Ferrari and Renault introduced it sometime mid-season 2015, having found out the previous year how far behind the game they were.
I offer an observation of my own, straying a bit off-topic here: the mass-market manufacturers such as Honda are in this game to explore the technology and train engineers, and what they're interested in is petrol engines that achieve both incredible thermal efficiency and reduced pollution profiles. Honda is unlikely to quit just because they're not winning. Alonso has a reputation for changing teams in ways that sabotage his career instead of helping it; in pursuing his it's-Honda-or-me policy with his present team's management, there is scope for him to score his biggest own goal yet.