I think Sandwhich makes some valid points.
The changes lose alot of synergy that non avatar hamelin had in spades which is what made him deadly.
The focus for the most part was attrition and not usually via killing the enemy but by making them worthless, at which point Hammelin can put more of the focus on himself forcing the enemy back while his swarm does the job for him. Again once that starts working and blight counters kick in he can go nuts with the disease etc.
Now however it all seems a bit disjointed.
In a nutshell he goes from basically being an arrogant bully who manipulates the board via insignificant placement and rat swarming to a model that has counter synergistic abilities in relation to the swarm, has powers that relies on the rat yet he loses out on the swarming ability because he can't summon more stolen or rats as well. (Yes other models give voracious rats but Hammelin was always the key to a large degree)
So he relies on the swarm as stated, can't keep the swarm going as well (and even requires perhaps an errata because of this) and works on blight counters which again is swarm related or Hammelin centric but he just aint as "meaty" as he was to deliver those blight counters.
Saying that this is "such a change to his game that it takes away former bad machups" is simply not true because when he manifests he still wants to do alot of the things he used to do, in some cases he does them better there is no doubt but when that synergy that A leads to B then skips C and leads straight to a D is what is lost. Now going from A to B is alot harder to do.
It is a very rare case that after the work you do setting up the first 3 turns or so that you would truly want to stop and switch things up halfway through when the switch really is a bit to radical and the payoff not worth the same benefit towards what you were setting up for.
Sandwhich is not truly theorymachinging as he is stating facts, I agree with some posters that you can't take Hammelin on his own and simply look at him in a vacuum because once again the key was the Synergy Nix, the catchers, wretched and stolen all brought together while using hammelin, his Avatar really does give up alot of that.