Art of memory:
was worked a lot in 4th edition. At least in playtest edition, i dont remember the edited version xD. I dont know if i can speak about that, there was a mystery related about art of memory with some nice spells and effects.
Flawless magic, is big but you will have only one major virtue and there are at least 3 great: Flawless magic, Flexible Magic and Gentle Gift. There are more than numbers in a game, and gentle gift can be game breaking. Try to start a spring covenant from 0 with 3 lab rats with flawless magic: 1 specializated in ReVi, other in CrIg and the third in PeCo, all with Blatant Gift and other social handicaps. Lets see how they will got time to investigate when they are fighting for survival. Only a well stablished hermetic market (changing vis for gold, tractatus and summae for vis and gold) will save then.
Flexible magic: is a great virtue as presented, and feats well in many concepts. And the same can be achieved with mutatum magic using vis, and yes, there are a form to make the flexible magic effect but with mastery and vis.
With flexible magic you will have at least 1 or 2 more spells in starting, you will make personal versions of all touch spells and still cast on other, or use on you with your parma active all touch spells, this is obvious. Also you can use with arcane conection range all your visual spells, also obvious, but really usefull. And to create many spells you have, at practical effects: +10 to lab total. Do you really think is a nerfed virtue?
Vulgar Alchemy is great. With vulgar alchemy and a verditious friend you can make a Warhamer with +5 to atack, and this is not so hard. The posibility to 'discover' new bonus is all you need for a minor virtue, but this do more. In 4th version this was really really overpowered, abusive.
Magical memory is good in some concepts. Not all virtues are as good as cautious sorcerer or inventive genius. Skilled parens is better than warrior and both are '+1' virtues. Allways you can retrieve the +x virtues concept of 3th and 4th edition if you are a power balance fan.
diedne magic is good for counterspelling, with a consistent minor focus can make a good specialist in counterspelling. With others focus, cautious sorcerer and life boost can be a bizarre hoplite.