I'd tend to agree in the main, but, purely speaking for myself, when I was looking for a boat I had a list of specific things I was looking for and then another list of 'preferred things'. The boat I found ticked every item on my must-have list. Should I have rejected it due to the engine? In reality, a BMC engine is likely to be far less painful than a Bolinder for a newby boater and mine had clearly been well cared for. The only reliability issues I've had, have been with the alternator and the starter motor, not with the engine itself. A badly neglected modern Japanese engine may well have given me more trouble and it's not always clear what issue may be lying in wait when you're buying.
It's certainly well worth having a good look over an engine and running it hard for a time to look for issues. I remember a former member (now banned) posting a video of his bmc 1.8 starting up from cold, and I was shocked by the clouds of smoke billowing out from the exhaust. I told him that there was clearly something very wrong with his engine but he was having none of it (he was a rather 'forthright' fellow - hence the ban). He insisted that all BMCs smoke like that. Well, they don't. I suppose what I'm saying is, there are good and bad examples of all engine models, it can be hard to know what you've got at first. If a boat ticks all your other boxes, then I'd not worry too much about engine type. Things which were more important to me when buying included: hull condition, length, insulation, internal layout, stern style. These things are more difficult to change than an engine, if they're not right for you.
If I was buying a brand new boat now, I'd have a preference for a modern japanese engine, but if I was looking in the used market, I'd be looking for the best boat I could find which suited my needs. Engine model would be only one of many variables.