Molly was also mentally/emotionally fragile, and (as an unstable mind-mage) incredibly dangerous to her friends as well as enemies.
And she was getting worse, not better, until Harry came back.
This^^. Though to be fair, in the case of the Ragged Lady it
was partly Harry's fault, because of Molly's involvement in his death scam.
Overall, Harry was an excellent mentor. Yeah, Mab and Lea thought he went to easy, but they
would think that. That's Winter. They'd have thought Justin went too easy on Harry and Elaine.
Morgan had a little more of a point, but you have to look at it in context. When Harry first became her Master, Molly had just barely come through a nightmarish experience that she had in no way been ready for. She had effed up two minds, but with good intentions. To complicate it, Molly almost surely
did save Rosie's baby from being born addicted, if not worse.
She had specific magical talents that especially lend themselves to misuse. I can't prove it, but I would hazard that more warlocks come to warlockhood through mind magic than from all the other Laws combined. It's just so easy to misuse, and so tempting to do so, and so many ways good intentions can lead to disaster.
Molly was full of adolescent rebellion as well, something Harry was easily young enough (and similar enough in personality) to understand, but she longed for her family, too. Harry recognized in her almost right away that however blase and independent she acted, her parents' good opinion still mattered to her, esp. Michael's. Remember how upset she was when she found out she'd still be living at home while an apprentice, and how happy she really was at that.
Now this same rebellious, confused girl thought she was in love with Harry, which complicated things enormously. (While she may have come to the real thing later, at that point it was infatuation and fantasy. Harry poured cold water on that, literally.)
Now keep in mind that part of her probably still thought she hadn't done anything all that wrong with Rosie and Nelson. Yeah, she's been told why it was bad thing to do, but at this point she hasn't seen how bad this sort of thing can go. She did get them off the drugs, and as I noted above, she almost surely saved the baby from being born a drug addict, or worse. So she did a bad thing with good intentions, and actually did get some good results (partly by luck, admittedly).
At the same time, her own motives were mixed, and Harry knew he had to help her sort that out if she's going to avoid being beheaded for a repeat offense.
Harry isn't trying to teach Molly to be a battlemage, he's trying to teach her to be a functional human being and a Wizard (as opposed to a warlock). At that stage, Molly could be compared to old-fashioned sweating dynamite, or the like. It would be very very easy to detonate it, to the disadvantage of everyone.
Harry did it. He managed to teach her to combine the morality she learned from Michael with her own situation. He taught her self-restraint. He taught to recognize that 'having the power' is not the same as 'having the right'.
Remember in one of the side stories, Molly is tempted to do something she knows she shouldn't, but then she remembers Harry telling her that the best way to deal with 'it would be wrong but' is to stop the thought at 'it would be wrong'. He reached her. She learned.
Now, eventually, she reached the point where he probably needed to start getting tougher with her training. That comes hard to him, and he might have been a little too easy-going at the end, just before
Turn Coat and around the time of
Changes. If everything hadn't gone to crap right then, probably Harry eventually would have realized that himself, but everything went to crap just then.
It's true that pain can be a good teacher. Sometimes it's the only teacher. It's true that a drill sergeant who is too easy on his trainees is not doing them any favors when war comes. But Molly had only just reached the point of being ready for drill instruction when things went south. Before that, she was a broken, fragile girl armed with a built-in mental nuke.
Mab and Lea, if they had been in charge of Molly's training after
Proven Guilty, might very well have produced a badass battlemage. Of course, she'd also have become a monster and almost surely at least four kinds of warlock. Or maybe she'd have suicided from overload before she was ready. Mad and Lea would just shrug at either. That's Winter.
So yea, overall, I'd say Harry was a
damned good mentor. He was the right Wizard for the job at the right time. Maybe, probably, toward the end, he needed to be a little tougher. But try that much sooner than it happened and you'd break Molly, one way or another.
So give Harry an A- as an instructor, esp. considering that this was his first effort as a master.