After testing extensively, my opinion is RF is viable and better for farming at higher MPs and lower MPs. Using Mantico.
First of all, you need to have at least hatred generation of 9 per sec with Bat Companion activated. High critical chance is recommended. If possible, get a good SOJ and DML with RF CC bonus. Game play is the most important. Don't miss it at the end of this thread.
Grenades * Thinkerer - Shoot 3 grenades dealing 95% dmg each is not the main reason for this skill. The ability to generate 6 Hatred is. Hatred pool is 150 so in theory 8 shots of 3x6 will fill up the pool provided all grenades hit the mob. This should be your secondary weapon.
Bola*Bitter Pill - You may also use this skill to regenerate disc with nice dps.
Rapid Fire * Bombardment - This is your main weapon. With 150 hatred pool, you can channel 25 seconds continuously provided there is no break up. 414% dmg and AOE of 8 yards is arguably the most powerful skill currently available to DH. Game play is important. I will explain later.
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Companion * Bat - 3 Hatred generation per sec.
Vault * Tumble - You need this skill to get out of sticky situations.
Shadow Power * Blood Moon - Don't think this need further explanation. Bood Moon rune increase life steal to 25% cannot be overlooked. You can also use rune Night Bane for additional 3 Hatred per sec but sacrificing additional life steal or Gloom, which ever suit you better.
Rain of Vengeance*Beastly bomb - The massive damage cause by the bomb is huge. Cool down of 30 sec is acceptable.
Sentry * Guardian Turret - Extra dmg and protection. You can always use other skill for this eg Preparation. This is the 1st time I opted out on Preparation. Will explain in game play.
Passives skills :
Nightstalker - Basically for Perma Gloom. Worked wonder with RF now.
Vengeance - Max hatred +25 and gain 20 hatred and Disc when heal by health globe.
Perfectionist - Reduce 10% Discipline costs and increase all your defenses by 10% including Life, or
Steady Aim - A good passive because once you start channeling further than 10 yards, the 20% additional damage will continue to kick in even though enemies are moving closer than 10 Yard provided you don't break the channeling.
Archery - Plus 50% CD for Mantico
Cull The Weak - This is also an option with 15% additional damage provided you use cold element SOJ, Buriza and Azkaranth on Scoundrel
The main beauty of RF is the ability to channel at amazing speed and crit hits. A full 150 hatred pool will allow you to channel non-stop for 25 Sec (estimated for Mantico). Imagine what dmg you can deal with rapid 414% weapon dmg for 25 sec. Don't worry if some of the grenades miss the targets, just don't break the channeling or else incur 20 hatred again. Always remember this, your Shadow Power * Gloom when activated before firing RF will always remain active even after 5 seconds. If there is no break in your channeling, SP will last for 25 secs. Don't waste Disc and activate SP when you see your character is without tentacle. SP is still working. How wonderful! That is why I don't need Preparation anymore.
When confronting elites or mobs, 1st activate your SP and shoot some grenades or Bola (if u use this) to steal some life just in case, then either sentry or beastly bomb before you start channeling. Whenever you are low on hatred, activate SP again and shoot grenades. The hatred pool will be filed up in no time (8 shots plus 9 hatred per sec generation) because there are also Health globe lying around which also fill up your hatred and disc. Make sure you have pick up radius bonus. Make sure you hit SP before channeling again. You will feel invincible by now and a bit tanky.
The problem arise when encounter with elites which can break your RF channeling eg Knockback, Voltex and etc. Don't panic, make sure that every time this happen, you need to reactivate your SP before channeling RF and while channeling don't waste Disc by pressing SP again.
If your try this out, I hope it will work for you because I m having fun time with this build.
Thanks for reading.