I remain optimistic if only because I haven't seen a fully optimized shadow or slayer in blood gods going against the top glads. I still have some glads that aren't fully built with strat/gear/skills and they are barely staying above 50% in blood gods. Seraph Zodak was like that too until I finally finished his build - that little sliver of difference made him a consistent top 10 fighter where before he was 50/50. We all know the margin of difference between a 80% win fighter and a 50% win fighter is pretty thin.
For Slayers - I agree they have too many activated skills, so it may make sense to try to minimize that in order to get the skill triggers you want. But I still think there are ways to build them with boosts, ranks, strats that will make them really tough. If you only look at their skills, I agree there isn't much to work with. But if you mix in the possibilities of added ranks like armor pen, block, crit, etc... I think they can be made to be very effective.
For Shadows - I also think they lack offensive punch and the shadow form skill seems difficult to maximize. But I think this is because managers haven't figured out the best way to maximize these skills and the right blend of stats, gear, ranks, strategies to be dominant. Think they could also be dominant with the right blend.
Both Shadows and Slayers have the essential ability to defeat tanks - shadows with spectral touch and slayers with blood bath. So the potential, at least, is there. We just haven't seen a manager creative enough to take advantage of it yet.