The reason there's a soft cap in the NBA is precisely to promote team-player continuity. If the owners have their way and a hard cap is imposed, then it's obvious there will be less team-player continuity and players who stay with the same team for their entire career will be even rarer.
The reason players are against a hard-cap is because teams will operate more on a boom-bust logic. Retooling and sustaining success will be a lot more difficult to do (think the Spurs that built 2 championship teams around Duncan, the Pacers throughout Miller career or Dallas with Dirk, the Pistons keeping their '04 title line-up for new runs... cases that won't happen under a hard-cap scenario). It'll be more like Miami last season. There will be very little incentive for non-contending teams to offer multi-year contracts to role-players. Which is basically a salary cut, as guaranteed wages are a premium. The same % of the revenue + a hard-cap equates a salary cut for role-players relatively to star players (that wouldnt' see their situation affected). As the large majority of the union members are role-players, it's obvious they don't like it and rightly so.
Plus, if owners get their way and the % of the revenue is reduced in any significant way, then I'd expect star players to gravitate towards certain locations - as non-salary factors will become much more important. Not sure if that's good or bad for Boston - it's certain terrible for Minnesota and great for LA.