It's only a small number very close to the site of the proposed new station who seem to have objected. I think you are correct, that based on experience elsewhere, in most cases house prices are likely to increase. The other concern voiced by objectors is parking given that many of the houses near to the line have very limited parking now.
I would have thought that houses close to the proposed station would have the most to gain in terms of value should the line be up and running. Of course, it would be undesirable if the streets became a massive parking lot for commuters from outlying districts of Portishead - something that could be solved by having residents only parking permits? Alternatively, perhaps the local buses could be included as free travel in Portishead for those with a rail ticket?
I note that the motorways in the Bristol area regularly jam up with traffic criss crossing the country meeting locals doing short hops. The rail link would at least remove some of these locals from the motorway network - especially if it was extended to North Filton and Henbury.
(It is regrettable that the M3/A303 direct route between London and Exeter is not a through road of motorway standard as the way things are, many people travelling between London and Exeter, Plymouth, Cornwall use the 30 mile longer M4/M5 route via Bristol. These routes already have to cope with the rest of the country - hence the jams in the Bristol area and Somerset. I see that the Government have had to cancel their proposed tunnel south of Stonehenge because a judge reckons it will upset the ‘ancients’. It apparently does not matter about people getting killed on the present inadequate road). Sorry to digress!