Parintachin wrote:
Dark Reaper: 2 attacks 3+ flown up in serpents, deployed behind 4+ save Scorpions or 5+ reinforced wave serpents win in my book, unless the opponent has no save at all. Most time I assault anyone, they're either in power armor (or better) or have cover saves, or both.
Dark Reapers don't have any extra attacks, they only get two attacks in shooting, not in assaults.
It's true that Warp Spiders are more specialised and are much more use coming out of the webway that any other deployment. Their value in a firefight really depends on their opponent, the kill differential (i.e. number of kills you get vs your opponent) is generally better than their 4+ FF appears because first strike and 4+ armour means your opponent won't get as many kills. They will average 5 first strike hits (assuming 2 exarchs). They are of course particularly good against small numbers of low armour, high firepower opponents, for this reason.
As for why they were changed, possibly they are a little bit too good compared to other aspects because of their armour, but I think the best reason is because it isn't really in keeping with their style of play - they're supposed to jump out of the warp. Swooping Hawks can't be transported by wave serpents either, for the same reason. They just don't feel right in a mixed formation anyway, especially when you use their scout abilities.