So, picking your master is based on approximately six factors: Your goals/familiarity, what you know about your opponent as a player, the strategy, the deployment, the scheme pool, and the enemy faction. Worse, your decision is based on an intersection of these factors that often reverse or significantly change the outcome! Lets take a moment to look at Reckoning (a Strategy Hamelin is traditionally bad at) against Ressurectionists (which is a faction Hamelin has significant problems against) and see how when you are facing Ressurectionists in Reckoning Hamelin is at minimum a top two pick, if not the best pick. This is because Hamelin has a hidden ability on his card that isn't printed: Hamelin dictates what your opponent's crew must be in order to win.
Ressurectionists are difficult because anyone who is across the table can and probably should reach for Datsue Ba and Manos. Lantern of Souls is going to shut off the card draw from Malifaux rats plus a few other things, and Datsue Ba means that Malifaux Rats are just Onryo that don't know it yet. However, both of those models have a significant disadvantage in Reckoning in that they are Henchmen, meaning that is a lot of Reckoning Points. Worse, one of the things Hamelin brings to the table in Reckoning is the fact that he can just eat Masters and Henchmen, meaning that they are not safe, and Manos definitely can not operate as freely as he may want to.
A Ressurectionist (or any faction, really) playing into Reckoning is either going to go for a small elite crew who can kill a lot, or include more "combat schemers" who can function independently and complete schemes normally, or go with pure scheme runners around a combat-centric core. A Ressurectionist is likely going to favor Seamus, Yan Lo, McMourning, or Jack Daw. You are more likely to see Seamus or McMourning into scheming pools that require a lot of mobility. Jack Daw is almost always going to play that small elite block that prefers schemes they can complete while killing you. I'm not familiar enough with Yan Lo to really comment. Outside of Datsue Ba and Manos, your big worries are Drowned, Jaakuna Ubume, Goryo, Dead Rider and Kentauroi. Of those, Dead Rider and Goryo are the most accessible out of keyword.
Lets say they are Jack Daw. 20ss is a long way out of the way for Datsue Ba and Manos, for the privileged of feeding you Reckoning Points, so you probably won't see those. You'll see Jaakuna, Drowned, two Hanged, maybe a Crooked Man, and maybe a Goryo with GST if they want to be cheeky. Whisperer on Jack Daw, Jaakuna, Drowned, Hanged x2, and Goryo with GST is about what Jack could afford. A Hanged would probably be dropped for something that gives more scheming potential in this case. Except... you a) know your opponent is going to field that and b) know that if they don't field basically that or something like it, then Hamelin can overwhelm them. Against Jack Daw you really need aoe, Focus, and anti Terrifying. You can also be reasonably confident they don't have anti demise. So something like Hamelin, two Rat Kings, Obedient Wretch, Hans, Hodgepodge Emissary as well as Ashes and Dust can work. Purchasing Malifaux rats in place of a Rat King is also a solid play that lets you have vermin early, but runs the risk of the Goryo. This is a small elite crew, but both A&D and Rat Kings can switch hit to being scheme runners when you need it. Most of your models are capable of two-shotting most of Jack Daw's Crew, or one shotting after some Pustulent Tumors.
If they pick Seamus, Whisperer and Carrion Emissary will quickly follow. He desperately wants Focus +2 from his Crew, especially in Reckoning, so lets say two Nurses follow that. Dead Rider and Goryo + GST are the biggest anti Hamelin models that don't run into the henchmen issue and... we are at 48 Stones. Practically speaking, that probably means down GST on the Goryo and/or a Nurse. The other issue is, even two anti Hamelin models doesn't really feel like enough, and you still don't get anti demise without feeding Hamelin points. So as Hamelin, what do you do? First, you are going to have to bite the bullet and hire Henchmen--specifically, you really want Nix and you want Pride. That is going to introduce some serious unreliability into Seamus' peekabo tactics thanks to lowering the maximum damage to 6 instead of 8, and forcing Seamus to cheat last is going put some serious pressure on his hand. Expect Seamus to want to go first and immediately delete Pride before Pride can go, so Pride is going to need Soldier for Hire. Emissary is excellent in this matchup because he requires at least 3 AP to kill, even in ideal circumstance, and he supplies a significant amount of healing. Obedient Wretch, who is going to survive through sheer force of Stealth, will bring us down to 18 Stones. We need a huge cache because of our Henchmen, so we are looking at Hans with Soldier for Hire, Rat King (*maybe* two), Ashes and Dust, or Malifaux Rats.
Lets say they pick McMourning--now you are in trouble. Dead Rider, two Kentauroi, and two Nurses is a fairly standard McMourning core that leaves McMourning with 11 Stones and is going to put a pretty solid krimp in your style. I would probably equip then Kentauroi with the Whisperer to capitalize on the card draw and give them a defense against Bleeding Disease. I'd also swap a Nurse to Rafkin, putting us at Rafkin, Dead Rider, Kenrauroi + Whisperer x2, Nurse, and 5ss. This list is very fast, heals a lot, and can swap between focusing down one target with poison and pumping out the AoEs to keep the rat population down. Hamelin immediately takes Obedient Wretch, Hans, Hodgepodge Emissary, and Johan Creedy. From there I'd either say two Rat Kings and 6ss or one Rat King, Nix, and 6ss. Henchmen are always a risk, but you desperately need aoe Blight and McMourning isn't so great at putting someone like Nix down. Hans tries to Stun + Distracted the Kentauroi with their Df 4 to keep them pinned down (seriously, Stunned *really* ruins their day). Johan is on emergency poison removal duty. Hamelin tries to isolate models that have already gone using lure before letting his crew have their way with them.
In all of these matchups, except possibly McMourning, Hamelin has the advantage. The Ressurectionists can't bring their strongest anti-hamelin tech to bare without putting themselves at a serious disadvantage. Rat King are simultaneously anti schemers, schemers, and beaters. You draw a lot of out of keyword and versatile models, but Outcast has a lot of strong models in the elite beater category to draw from. You could argue Leveticus is better in all of these cases, and that might be true. However, its close enough that familiarity and scheme pool are going to tilt things in one direction or the other.
In contrast, if you try to play Hamelin into Ressurectionists on Cursed Idols you... ...