Been a while since I had the time to do one of these breakdowns but here's an attempt at giving each a fair shake under three considerations(there is no personal stategic as the spell simply can't have a strategic impact if only Mathilde uses it):
-Tactical(General) - The per-battle impact, as used by any Grey Wizard
-Strategic(General) - The per-battle impact, if usage is proliferated
-Tactical(Personal) - Mathilde-specific tactical advantages over and above the general one. Note that Boney specifically mentions that apparitions can/must be trained to do their jobs, meaning Mathilde's personal relevant skills matter here.
[ ] [RIDER] Mounted Wraiths
-Tactical(General) - The appearance is a standard issue binding for Necromancers, and we can expect the associated morale effect of common infantry fearing it. It may be valuable for baiting out the wrong counters, since they appear undead but are not...but it only works on non-wizards, since Windsight will have the gig be up rather quickly.
-Strategic(General) - This is rather PR unfriendly, since it makes you look like a necromancer. While those with Windsight will soon realize that they aren't actually undead, it will raise rather uncomfortable questions if keeping the Apparition nature of the spell secret is a priority, and even more so for the lady who owns the original Liber Mortis.
-Tactical(Personal) - Ironically the Liber Mortis bonus to commanding undead armies SHOULD apply here, because Hexwraiths are a common Necromancer minion.
[ ] [RIDER] Mathilde on a Shadowsteed
-Tactical(General) - The apperance of a Wizard Lord on the field forces the opponent to deal with it, and so unlike other summon options, this would cause enemies to run towards it. They cannot afford to assume it is not Mathilde - because Mathilde has a bit of a reputation for appearing out of nowhere, ruining your day and then vanishing. They can't see through the deception with Windsight because it'd be a beacon of Ulgu...which is exactly what Mathilde would look like to Windsight anyway. By the time they're close enough to check her gear its too late.
-Strategic(General) - This further expands our urban legend for good and ill, since theres already a seed story in Stirland. Now, for the most part Grey Wizards would tend to be doing things we generally approve of, so it should be a benefit in general, but both miscasts and enemies who got their day ruined by Suddenly Mathilde Weber might catch up to us later.
-Tactical(Personal) - As this is exactly Mathilde, she has the additional option of using it as a personally specific decoy. In addition she can train it to fight like she does, though actually getting her full sword style would be challenging, she does have both a lot of sword traits and mounted combat traits that would be applicable to getting it to the right skill level.
[ ] [RIDER] Generic Sword Wielding Grey Wizard on a Shadowsteed
-Tactical(General) - An extra grey wizard on the field doesn't demand attention like an extra Wizard Lord on the field, but can't really be ignored either because thats how you get Pendulum'd. They can't see through the deception with Windsight because it'd be a beacon of Ulgu...which is exactly what a Grey Wizard would look like to Windsight anyway.
-Strategic(General) - An extra Grey Wizard showing up to a battlefield that already had one Grey Wizard on it isn't very notable, other than possibly inflating estimates of College manpower.
-Tactical(Personal) - As this is ALMOST Mathilde, she has the additional option of using it as a personally specific decoy if she slaps on a Doppleganger or other disguise first. In addition she can train it to fight like she does, though actually getting her full sword style imported would be challenging, she does have both a lot of sword traits and mounted combat traits that would be applicable to getting it to the right skill level.
[ ] [RIDER] Knight
-[ ] Conventional Empire
-Tactical(General) - An extra knight on the battlefield doesn't really force a unique response. However its an apparition. A supernatural murderblender that is largely expendable. Reacting to it like its a regular imperial knight is going to get whatever formation it slams into brutally killed.
-Strategic(General) - Knights appearing out of nowhere and kicking ass is mostly normal and unremarkable, especially if theres no noticable coat of arms.
-Tactical(Personal) - Mathilde has the skills to train a fairly basic mounted sword knight, but it'd be relying more on being a supernatural beast than anything else.
-[ ] Winter Wolves
-Tactical(General) - A Winter Wolf knight should be approximately the right asskicking level, and enemies would likely respond with an appropriate level of force. Right propa intimidating though.
-Strategic(General) - We may be on good terms with the Winter Wolves but I suspect if this version proliferates they'd probably start asking questions like where these winter wolves come from and why are they fighting for causes the Order don't know.
-Tactical(Personal) - Mathilde knows a fair bit about mounted combat and Wolf can probably help with getting the appearance and animations correct, but otherwise not much she can specifically train skills into.
-[ ] Demigryph Knight
-Tactical(General) - A Demigryph knight should be approximately the right asskicking level, and enemies would likely respond with an appropriate level of force. Right propa intimidating though.
-Strategic(General) - Demigryphs don't exactly grow on trees, but the lack of apparent affiliation should keep things confused.
-Tactical(Personal) - No unusual personal advantage.
-[ ] Winged Lancer
-Tactical(General) - Winged Lancers have a decent morale effect that should work fine here, though it takes more than one to really make it work.
-Strategic(General) - I suspect if Kislev was better put together they'd be asking questions about who's doing silly buggers, but from what we know right now, nobody would be in a position to care until its a fiat accompli.
-Tactical(Personal) - No unusual personal advantage.
[ ] [RIDER] Spider
-Tactical(General) - Giant monster spider is pretty scary on general principles, and I believe some of the Goblin tribes make use of Ulgu, so it'd even sort of fit if they had magesight to validate with. Its a decent deniable summon in wilderness areas, though more difficult to use alongside allies. Combat wise...its a big monster and fights like a big monster, so the level of force baited out would be approximately correct, other than not having poison.
-Strategic(General) - Its a giant spider, so what?
-Tactical(Personal) - No unusual personal advantage.
[ ] [RIDER] Centaur
Not familiar enough to give a fair assessment.