OK. Then assuming that one wishes to keep Diedne and Life-Linked in place, to allow mage types that specialize in spont and eschew formulaic and ritual, what kind of built-in Flaws would you suggest to attach to their systems, to balance things in your view ? Poor Formulaic ? Rigid ? Unstructured ? Something else ? I might see that a mild houseruling might be warrnted, but IMO either banning these Virtues (as well as the likely-to-resurface 4th Ed. Faerie-Raised Magus, which reproduced the effects of Diedne, and is likely to reappear in RoP: F) or making Diedne Magic a "kill on sight if discovered" Flaw are wholly unacceptable.
Differing opinions about it. IMO no more unbalancing than Flexible Formulaic Magic, Flawless magic, Life-Linked Spontaneous Magic, Entrancement, Shapeshifter, Major Magical Focus (Damage), Faerie-Raised Merinita (Faeries), Augury and Theurgic Casting (that Theurgic Virtue by which you only need to know a Vim spell for each form, to summon a Magic spirit you can command to produce any magical effect you like) from Mysteries, Cursing, Wonders and Intervention from RoP: Divine, etc.
As I said above, IMO it is no more unbalanced than half the list of Major Hermetic Virtues, or the right combination of 2-4 Minor Hermetic Virtues. Or some Major Supernatural Virtues as well. Be an Ex Miscellanea, pick Entrancement and Shapeshifter: gosh, there's two Forms and a Technique's worth you almost do not need anymore for typical uses.
Pick Flexible Formulaic Magic, and a smattering of basic spells in the most important Techniques and Forms, specialize in 2-3 Techniques. Who needs spontaneous magic then ??
So ?? The Diedne magus that trained the half-trained apprentice that was adopted by another house and trained the master of my master is long dead. I am as legitimate as the Tytalus magus there, whose entire lineage a couple of generations ago was marched as diabolists, and that Ex Miscellanea, whose grand-grand-master was a follower of Daimoh-alladh, and that Flambeau, whose master was Renounced for diabolism as well....
Questionable lineage is not an Hermetic crime. Once you have been sworn in the Order, all that matters are the violations of the Code YOU perform, not your lineage.