I've never had any major problems with Mr. Fixer. He ends up being about average in terms of being useful with each fight I've used him in. My favorite thing about him as how his tools let him be versatile with his damage application.
As for Absolute Zero, I've always seen Nightmist as being an overall better version of him. Bryan has AZ's strategy down pat, but I feel that Nightmist can be much more offensive once she has the cards she needs on the table. The one major advantage AZ has over Nightmist is that he has more ways to search his deck for those needed cards, so even though Nightmist is the superior character, imo, she requires more patience to get those cards out of your deck that you want. Once she does though, she's a beast.
As for Absolute Zero, I've always seen Nightmist as being an overall better version of him. Bryan has AZ's strategy down pat, but I feel that Nightmist can be much more offensive once she has the cards she needs on the table. The one major advantage AZ has over Nightmist is that he has more ways to search his deck for those needed cards, so even though Nightmist is the superior character, imo, she requires more patience to get those cards out of your deck that you want. Once she does though, she's a beast.