The number of head clashes in modern day football is alarming, it goes with the vastly increased pace of the game. Those pose a major threat, as do tackles with a lot of force, even though many/most(?) fans may say the tackler won the ball. Those tackles, with the speed these days, are rightly being outlawed.
As for heading the ball Glasgow University did a massive study and found that, yes, there was an increased likelihood of dementia-type illness in later life for footballers. This was offset by their lower chances of serious illness or death in other major diseases that affect the general population.
The dementia element is often quoted in the media by campaigners. What they don't mention, though, is that the level of dementia in goalkeepers was just the same as it was for outfield players.
I think it's safe to say that the results of that huge study were inconclusive.