A lot of people refer to Serissu as the Scum & Villainy Howlrunner, and they are right. To a point. But I am seeing a lot of lists that make it clear most folks aren't aware of the subtle differences, or if they are they have yet to really explore list building and tactics with those differences in mind. So, here we go.
The first significant difference is that Howlrunner boosts offense while Serissu boosts defense. The effect this has on list building is not obvious at first glance.
Howlrunner wants to be surrounded by a large number of friendly ships. Every additional attacker increases the value of her reroll. If the number of friendly ships gets too low (less than 4) it becomes questionable whether her ability could be replicated more efficiently through directly upgrading those ships. The result is that you see Howlrunner as a very strong and near-universal addition to Imperial lists with 5+ ships, and rarely if ever in 2-4 ship lists. The other interesting note is that her effectiveness is completely independent of the number of enemy ships, it only matters how many friendlies are shooting. She is included based on the composition of your list, independent of the meta.
Serissu does not care how many friendly ships she is near. Repeated for emphasis, Serissu DOES NOT CARE HOW MANY FRIENDLY SHIPS she is near. Her ability gains effectiveness based on the number of ENEMY ships that are shooting. Even if she is only near a single friendly ship, that ship will still get the benefit of her reroll every time it takes fire whether that is one time or 8 times in a single turn. She will be most effective vs. swarms, least effective vs. HLC Dash, Phantoms, or any other list that depends on big, highly accurate but low volume shots. This makes Serissu much more of a meta choice, her inclusion is based on what lists you expect to see and what your list needs help against, but she can be included in ANY list you have if you decide you want help vs swarms.
The other significant difference is that Serissu has the potential to be very powerful offensively, through use of the Heavy Scyk title, whereas Howlrunner is already very powerful offensively. Care should be taken, however, because at Serissus base value of 20 points she is already an efficient target for your opponent. Any offensive upgrades you add make her a better initial target, and you are unlikely to get full value out of them. To keep her alive and get the most from her, you are better off surrounding her with ships with very high offensive values. The same is true of Howlrunner, except that the nature of her ability rewards a large number of ships, which is why she is almost universally the first ship targeted.
All of this means that Serissu can be a valuable addition to any Scum list that has the 20 points available for her, but will be most valuable surrounded by larger, more dangerous ships rather than a swarm of small ships like Howlrunner is prone to. Where 5+ ship formations is the norm for Howlrunner, Serissu will find her home in 3 or 4 ship builds (maybe even 2, though an 80 point scum doesn't yet exist).
Finally, one other thing to keep in mind. While offensive boosts are generally a bad choice for Serissu, if you are already planning to include her anyway she offers a very inexpensive way to add some utility to your list. My favorite utility load out for her is 5 points, Veteran Instinct, Heavy Scyk, and Flechette Cannon. The stress is universally helpful without being such an enormous offensive boost that she automatically becomes priority target #1, and combined with a PS 10 she can serve as your anti-Phantom counter (something Scum struggles with across the board at the moment) by stressing Whisper/Echo before they can recloak.
