The chances of success in learning a ritual are prohibitive and, to make things worse, the DP cost of acquiring even a single ritual spell is high enough that it makes more sense to just buy an Additional Profession for 20 DP (or Arcane Power or Arcane Circle for 30 or 25 DP respectively), especially if you're a Human with the 5 DP Profession Adaptability discount.
One of the examples in CoM is a Ritual for 19 PP worth of Major Healing. The Final PP cost of the ritual is 38 PP (2 x the PP cost for a spell-caster casting it as a spell). This "Final PP Cost" affects both the chance of learning the ritual AND the chance of successfully performing it.
If the maneuver roll to learn the ritual succeeds, it then costs 19 DP to learn the ritual and another 2 DP (or 4 if not favored) to buy another rank in Ritual Magic.
Actually performing the ritual is even worse. To start with, it's a -40 sub-skill. And while there is a +5 bonus for each participant in the ritual (+10 if they've fasted and abstained from magic for long enough), there's also a "-5 x (Final PP Cost - number of skill ranks)" penalty for EACH participant. e.g. if a participant has only 20 ranks in Ritual Magic, that's a -90 penalty, just for that single participant.
I can sort of see this penalty as being justified for the leader of the ritual (although I think it should be the actual PP cost, not the doubled "Final PP cost"), but not for the skill ranks of other participants.
If you take rituals in movies and books as a guideline, with a High Priest leading a bunch of nameless mooks, most participants are going to be 1st level at most, with a maximum of 6 ranks. That's -160 per maxed out 1st level participant. Oops, sorry, that's only -155 after the +5 per participant bonus. Maybe that's why evil priests have a nasty tendency to just sacrifice their acolytes in rituals - it's the only way to get any benefit from them when summoning Cthulhu or whatever.
Even if all participants are player characters, it still means that adding participants is far more likely to make performing it harder, not easier - especially since very few players will be willing to spend, e.g., 76 DP (or 152 if non-favoured) to get 38 ranks in Ritual Magic just so they can help with a Major Healing ritual. Every single one of them would be much better off with a level or two of Cleric or Vivamancer so they can learn the Major Healing spell (and Mystical Arts skills like PP Dev and Cantrips/Runes/Alchemy/etc, and a bunch of other spells while they're at it). And the party as a whole will be better off if everyone can cast Major Healing in an emergency, not just the party healer.
Even taking 19 PP of Major Healing as Blood Magic makes more sense.
Has this ever actually been play-tested, let alone played in a real game?
I like HARP a lot, it's my favourite RPG rule set, but there's a lot of stuff in it that seems like it's never been play-tested at all, not even by the person who came up with the rule.
It's almost as if the process is: I have a cool idea, let's make it work. Then, in the name of Holy Misguided Balance, let's make it suck so that no-one sane will ever use it.
BTW, the Arcane Dabbler talent in Harper's Bazaar Annual is also silly. Who would spend 20 DP just to get access to a single spell when Additional Profession costs the same? or when Arcane Circle costs only 10 DP more? 20 DP for only one spell or 30 DP for a whole sphere of spells? suck it up and pay the extra 10 DP, you'd be crazy not to.
