So I've played a few games with Hamelin now and have some thoughts I figured I would share about how he and his crew interact, and some things to avoid. My games and end results were as follows: Hamelin w/ 3 stolen, 2 rat kings, 1 rat catcher, obedient wretch, nix, a few rats and 6ss vs Nekima won 5 to 3. Hamelin w/ 3 stolen, benny, nix, wretch, 2 rat kings, 1 rat catcher, hodgepodge effigy, 3ss vs jack daw loss 4-1 . Hamelin w/ 3 stolen, benny, nix, wretch, 2 rat kings, 1 rat catcher, 1 rat, 5ss vs somer draw 3-3. (lists may be off by a few ss, didn't double check just typed what I remembered.
A few general tips I want to state before I get into a few nitty gritty things:
1) Turn three is when you should be swinging at full force and causing some massive damage. Surviving until then without falling too far behind in points can be the difficult part.
2) Don't focus to heavily on the rat engine (I'll bring this up again later)
3) STACK BLIGHT LIKE YOUR LIFE DEPENDS ON IT
4) Hamelin is pretty squishy when cornered, but the stolen offer some level of protection. Let them die when needed.
5) The crew bubbles to some extent, but if you work out your bubbles the right way you can easily divide and conquer.
The points I really want to discuss in depth involve how to bubble up your crew properly, and what I mean by don't rely on the rat engine. Most of the other points are self explanatory and a bit more obvious from the get go.
Lots of crews have to bubble, and Hamelin is really no different. My suggestion is to bubble Hamelin, Obedient Wretch, 3 Stolen, and nix together from the get go, and bubble up 2 rat kings, benny, and a few rats. Benny will normally break off when he needs to in order to go do other things, but starting him there is a good idea in order to not have him get singled out. When it's time to split up, the rat kings can slingshot themselves to some extent, and your other crew members can help out with that if needed. Hamelin needs to be with the stolen at all times, and leaving him with the wretch helps support him and provide an additional source of rats and blight. Nix can be used wherever, I've just preferred starting Nix with Hamelin and breaking off later if needed. These smaller bubbles keep the board from getting too clogged up, and allow you to separate your early game from your late game without risking being the target of repeated hit and runs.
Why the rat engine isn't all I had hoped it would be
It's really easy to get caught up doing wild summoning with all of your fun abilities, but they really aren't as great as they seem. Loyal rats with tiny hats eats up way too many of your schemes in a crew that can summon to scheme run efficiently. Use it when you have scheme markers that you aren't planning to use, or when you can target enemy schemes. If neither of these applies, avoid the ability. A usable scheme marker is not worth trading for a rat. Tangle together is probably the best summoning in the crew, as sacrificing 4 rats for a rat king is amazing, Setting it up can be tricky, as can getting enough rats close together without dedicating your turn to it. If it happens it happens, if it doesn't just keep playing. Rat problem is situationally better than tangle, as being able to kill an injured rat king and surrounding rats for a full health slowed rat catcher not only gives you an awesome new model but also allows you to draw cards. Just be careful using it, as it can unintentionally kill rats your forgot were there.
What to watch out for
Hamelin's crew is afraid of a handful of things. Terrifying is big. WP 5 only pretty much everything and WP 2 on rats is really tough, especially if you are low on cards to cheat and have been flipping poorly all day. In my game against Daw my opponent's hanged held position against a rat king, a rat catcher, nix, and 2 rats for 2 turns because I had very few cards to cheat and couldn't flip above a 7 for my terrifying duels. Armor and hard to kill are tough to keep up with early game since so many of your attacks that do decent damage rely on blight which takes some time to stack. Slowed and stunned are also difficult to fight through. The biggest thing when playing Hamelin is carefully selecting your schemes. Pick ones like harness the leyline or assassinate that don't require you to break up too much.
Hamelin definitely has potential to be a good master, but suffers from how wide the variety of abilities and potential strategies. Don't get lost in the rat engine, be careful with your scheme selection, and don't get too broken up and you should be fine.