My internet was down earlier today, so I did some SCIENCE:
Unless noted otherwise, the testing is C-whoever vs. H-VSion. 0% guard gage, max circuit, enemy set to guard all. I used A (guard break) A-41236C to test arc drives, and Jab (guardbreak) A, D (A, j.D where needed) to test all last arcs. Other notable special moves were tested with A (Guard break) C->move
Arc: Last arc seems to hit. Arc drive goes through the burst but doesn't hit.
Warc: Last arc animation happens, fails to hit. Gets hit out of arc drive. 623A will go through the autospark.
AKiha: Last arc comes out, can be blocked. Arc drive comes, goes through her, invincibles through the burst.
Maids: Kohaku last arc goes through, fails to hit. Hisui last arc seems to work? Can't really be sure. Both Arc drives go through spark, fail to hit.
Miyako: Last arc goes through but does not hit. Oddly, gets hit out of another arcdrive but not arcdrive. Neither hit.
Ciel: Arc drive gets blocked. Last arc gets blocked
Sion: Arc drive doesn't hit, Gets hit out of Another Arc, Last arc gets blocked.
Aoko: Arc drives get blocked, Last arc starts but doesn't hit.
Tohno: Last Arc misses, Arc Drives miss.
Nanaya: Last Arc Misses, Arc Drives miss.
Kouma: Last Arc hits. Is hit out of arc drives.
Ries: Last arc Miss, Arc drives miss. Notably, however, H-Ries's 63214B autoguards the spark.
VSion: Last Arc misses, Hit out of Arc drives;
Wara: Last Arc hits, Arc drives can be blocked. Notably, All 421 and j.421 moves go through the spark on cancel. j.421C has to be blocked. Wara's jabs at max range also evade the burst outright.
Roa: Hit out of arc drives, last arc misses.
VAkiha: Arc drive misses, Another Arc gets blocked, last arc misses.
Mech: Hit out of Arc drives, I find it funny that the stampede sound still plays. Last arc hits three times (First hit seems to miss, possibly the last hit too?)
Satsuki: Arc drives miss, Last arc hits.
Len: Gets hit out of both arc drives, but the normal one continues on and has to be blocked. Last arc cake comes down but doesn't hit.
Ryougi: Hit out of Arc drive, Last arc and Another Arc miss.
WLen: Same as Len.
Nrsodjgps- your mom: Arc drives hit, though I'm sure they can be dodged. Last Arc hits but I think a few hits miss in the beginning.
Hisui: Same as Maids ver.
Kohaku: Same as Maids ver.
Necoarc: Last Arc and Arc Drives miss.
All: You can heat activate through the burst.