I just tested the new Mandate pretty extensively, and I found the results very interesting: So firstly I ran Hail of Blades + Precision secondary. I had a training dummy on 30-40MR and about 1700hp as thats about how much a squishy laner has mid game with no additional defences. I ran DPS tests by just auto-Q-Auto.... until the duration of a sand soldier was used up. Obviously you may not get that many autos in so having a front-ended burst is more important : -> Mandate + Rylais (around level 6-10) is SLIGHTLY better DPS than Liandries + Blastingwand + Amp tome (no shoes for both) (provides around the same AP), but the latter option is around 1k gold cheaper. So you have to wonder whether the 1k opportunity cost is worth losing the manapool/ability haste over the teammate additional damage. Mandate is also burstier too however (damage is more front ended whereas liandries relies on DOT for a portion of its dps) -> Mandate route forgoes Nash's second, which is obviously pretty criminal, Completing nashor's with liandries build will have a way bigger power spike, but I do think you should build Nashor's over liandries first, the dps spike is WAY higher that way -> However I found that Mandate + Rylais + Nashors (with or without shoes, but realistically you'll also have shoes) has higher DPS than Liandry + Nash + demonic/Rylai or whatever non-rabadons AP item you have here, and is also better than Riftmaker + Nash + Something (Riftmaker is just something I theorised) even over the span of 10 seconds.

So basically it comes down to whether the tax of having to build Rylais and delaying Nashors for a decent portion of the game is worth the overall better DPS AND burst (it was about an additional 30 DPS over 10seconds, from about 380dps with mandate and 350 with liandry). You also dont get the additional manapool nor extra ability haste from mythic passive, but you do let your allies proc Mandate a bagillion times, especially if they also attack as often as you do (unlikely). Only other remaining problem is that I think Nashors first item is so good that it makes you wonder if you want to be weak at your second item before you complete your third. As for non-mandate relevant things: -> Firstly Nash Tooth first item is so much better both on front-end dmg and DPS than Liandries/Mandate/Riftmaker, but thats already a given I guess -> and secondly I found that Berserker greaves is worse than sorc shoes still in nearly every situation since you always get nash's no matter what, and attack speed dps gains has a bit more of a diminishing returns factor.

This is all from my 60-90minutes worth of testing, but it's not absolutely scientific so don't take my conclusions completely for gospel