En Suites are grim. I wouldn't even consider a house that didn't have a separate bathroom.
I've heard of someone who thinks indoor toilets are disgusting, he can't believe that some people shit in the same building where they prepare their food and eat. (Ancient American in an unmodernised house built in the era when it was normal for the toilet to be in an outhouse.)
I think some people have very odd views, based on insufficient exposure to the modern world.
Literally the only difference that necessarily exists between an en-suite and a separate bathroom that isn't is that you don't have to walk through a shared corridor to get to an en-suite. Being smaller, not having windows, smelly, damp, are literally faults any bathroom can have, when compared to another.
You can argue that being directly connect to the bedroom means it's easier to release smells and steam into the bedroom, but that's not a problem I've ever come across in the eight en-suites I've experienced in my life. Perhaps this is a problem with en-suites in conversions, I have no experience of those. The ones I've used were always built-in when the house was first built.
I do have a windowless en-suite in my London flat, it is identical in size, facilities and quality to the (also windowless) main bathroom, and doesn't release anything into the bedroom. The door is on rising butt hinges, so will mostly close itself even if I didn't deliberately close it behind me. It also has an extractor with a humidity sensor that runs on high setting for up to two hours after a shower, and on low-setting 24 hours a day otherwise, so the air is always as fresh as any bathroom with a window I've ever used. (Actually, the air is better than some windowed bathrooms...)