The problem with the Plyouth Brethren itself is that it's recognised as Chuch officially, but works as a cult.
A cult is defined by the control they have over their members, the way they manipulate their behaviours and a range of practices they do to keep the control over them.
Some of them are the authoritarian leadership (one single leader never questioned, considered infallible), members are separated from outside influence (they can't make friends with anyone who is not in the cult, or eat with them), they are taught to see outsiders as dangerous, corrupt or evil and depend on the group. They are not authorised to study in University, and normally they learn their skills inside of the group, but are given high salaries, and are threatened with the fact that if they quit the group they will also lose their jobs, and as they can be ill equipped to find another job of a similar pay rate, they will have difficulty, unless helped by organisations.
They are made to fear, they are punished and they are silenced and isolated if they skip the rules. They give up their personal autonomy and privacy, and their mobile phones normally have some software installed that will monitor and censor their online activity. They discourage critical thinking and questioning the doctrine is seen as a sin.
They are not allowed TV, recorded music (this has changed recently, but only reduced to music recorded by their musicians, and selected pieces), no reading of newspapers, no joining unions. They go to church almost every day, and on Sundays three times (the first one earlier than 7am). They are not allowed pets (who give good emotional support). They have very little to zero emotional literacy, as they have to supress their own feelings and desires to conform with the community. Many of them drink a lot of alchohol as a coping mechanism.
Women are not working other than in the house, so if they have bad luck with their marriage and they want to separate, it becomes very hard for them. If they remain in the cult, they are not allowed contraception (unless prescribed by a doctor for health reasons), they have very limited opportunities in life. On weekends, they receive people at their homes, but it's the church who dictates who is going to visit them, and creates a rota for it. So when you have a lucky day and receive the ones you enjoy, that's great, and not so great the day you have to force yourself to attend other people you are not so keen on, or simply don't have fluid conversation with.
People don't join the PBCC, they are born into it, so they never had the opportunity to decide if they wanted to be there or not.
They can't go on holidays to a hotel, and only with permission of the church, they can go only to be lodged by other brethrens. They must have a detached house, as touching walls with other people is a way to let the "sin" in.
Two brethren cannot divorce and remain brethren, so the one who is leaving is completely cut off their family, friends and in most cases, children if they have them. Members who leave need psychologycal assistance and support to adapt to living in the world and overcome cultic indoctrination.
But the PBCC has money. Money enough to pay for some conservative campaigns or to make donations, and they can send their members to leaflet for them, even if they are not allowed to vote themselves, because who runs the country is a "matter of God", but they got billionaire contracts for PPE and related services, raising concerns about the transparency and fairness in the procurement process, after cronysm and lack of competitive tendering allegations.
Now, once this is set... does it impact you if you buy the adyacent house?
In priciple, no. They have limited social interaction and are busy going to many church meetings, and won't make friends with outsiders.
Be aware of potential surveillance, as they can have monitoring tactics. Maintain boundaries, avoid discussing sensitive topics.
The only more thing I could add is that if you live beside a brethren family, and observe visible signs of distress isolation, restrictions of communication or indications of emotional or physical control or coertive control, you can report this to the local authorities (for immediate danger or suspected abuse). If you are that type of kind person, you can also have resources for ex-members at hand (local support gorups or organisations), and if you encounter anyone seeking help, that can be shared with them, without directly intervening. Sometimes a good neighbour in a moment of crisis can really make a difference.