Looks good Except for the Vitkir. I always found the vitkir concept really cool, but the rules to be useless: the system is more complicated than the rest of the whole ArM mechanics: too many runes. It seems that there are as much runes as in Ultima Thule, so vitkir do not cause a good feeling to me. Of course they might be pure flavor, and then I wilñl gladly eat my words. We will wait.
I ran an arc on Man that used a viktir in a village; his runes weren't too tough to set up...they're vaguely similiar to the arts and if you go at it with the idea that some viktir are better than others with specific runes, it's a lot less of a headache.
Medieval charm magic was very similar in form and function to Roman charm magic, aka defixiones from Ancient Magic, largely because it was derived from the same sources.
I found the Rune-Scripts in "Ultima Thule" difficult to use as well, but so long as the writers clearly define what each rune can and cannot do (like Alex White did so well with Hyperborean Hymns) it should be workable. The fact that the indices list guideline for each rune are a good sign in that regard.
Elementalist - Possibly none. Their ability to summon and control elementals is sufficiently difficult for hermetic magi that they may not need to 'break any concrete limit to compete with hermetic magi.
Folk Witches - My first instinct was the same as Fruny's, but it would make sense thematically for witches to be able to do all sorts of cool new stuff with appropriate arcane connections.
Gruagachan - Limit of the Divine is a good suggestion, but Limit of Time may also be a possibility depending upon the exact nature of their "Vision" powers.
Learned Magicians - The section headings aren't particularly useful, possibly something to do with the Limit of Arcane Connections?
Nightwalkers - Their abilities look pretty specialized (possibly why so many alternative traditions were included) shrugs
Vitkir - The Limit of Essential Nature when enchanting items as per Ancient Magic, but could there be more?
Under plain vanilla Hermetic magic, it's impossible to "summon" something without an Arcane Connection to that which is being summoned. The way to get around this is for the Hermetic magus to actually create the object rather than summon it.
While generally true, the rules in TMRE (vanilla ReX guidelines) together with the examples show that elementals could be summoned using a suitable element as an arcane connection (fire as an arcane connection to a fire elemental in the example given). IIRC.
True, but here we have the wonderful Ars Magica "When is an Elemental not and Elemental" problem. Elemental is used as a catch-all for spirits of elements, half-spirit-half-physical element-based beings and animated physical-element-based beings. From the point of view of the rules, a spirit of Fire, even one capable of manifesting a physical form, is not a fire elemental. A fire elemental is a living, physical fire. Which, I suppose, you could use as an AC to summon a spirit of fire.
Well, even if my all-mages PC saga (currently on hiatus) does not offer much use for companion characters, the presence of Hermetic Integration rules has just shot this book from "I'll get it eventually" to "must buy ASAP". Even more magical secrets for my party of Hermetic Indiana Joneses to quest for. Yummy.
They don't, at least not yet, but never say never. Mystery Cults of various stripes have been a recurring presence in our saga, mostly as opponents so far, but also as friendly contacts. Some of the players have expressed interest into MC membership, and the CoH would qualify for the cult type they would be interested in. As it concerns my PC, he wouldn't have too much difficulty with the CoH modus operandi, his magic style is already rather syncretistic. Anyway, the MCs that we met in our saga so far, are the Merinita, a homebrewn cult built around the Adamic language, the Green Cockerel and an infernally-corrupted offshoot of the same (demonic plan to swamp European economy with alchemically-created gold)