TL:DR - It Depends
Marker drones have some definite advantages and disadvantages.
Mobile - Both in movement distance and ability to fire markerlights penalty free
Durability - 4+ save with the ability to throw in a Shield Drone or two for that 4++ and 5+ FNP annoyance if targeted directly
BS - 5+ Base BS sucks, but a single drone controller boosts them to pathfinder equivalence
Cost - They are more expensive base than Pathfinders
Flexibility - Other than taking a couple Shield or Gun drones, they are very 1 dimensional
Savior protocols - Cannot be used to save themselves (I.e.-drone from a gun squad saving one from your marker squad) and you may be tempted to knock of Marker Drones via Savior if the enemy starts targeting something else nearby. Efficiency calculations for low strength 0 AP weapons actually say you are twice as likely to kill a drone by shooting something closeby than by shooting the drones directly if you choose to activate Saviour Protocols.
Leadership - Drones are highly susceptible to fleeing (for some unknown reason...I mean seriously...they are the most courageous unit in the game when it comes to taking damage for another model...but as soon as a couple die the rest are like "no thanks...we're gonna go back to sweeping floors and delivering Amazon packages.")
Many of these disadvantages can be mitigated (Drone Controllers, MSU, Common Sense). In the end, I've played about a third of my games using PF for ML and about the same using Drones. My play style is generally very dynamic, so PF have been less useful for me, but it all comes down to personal preference.
I will say, that the most frustrated my opponent has ever been with me was when I took 5x Drone units with 4 ML and 2 Shield Drones each and had them being controlled by a Stealth team in cover with a DC and 2 Shield Generators. He tried shooting up my drone squads first, dedicating a ton of fire just to chew through my Shield Drones in a single unit. The next turn he spent another couple units to kill the unguarded Marker Drones then went directly after the Stealth Team after he realized he'd only taken away 4 ML from my force. Needless to say, the Stealth Suits were fine.
To secure victory, the wise must adapt ~ Puretide