Since it's compatible with HC then it's almost guaranteed that it be as hard as mod as HC.
However, certain people are thinking that this makes the game unmodable and that's not the case. It depends of what kind of mods you are looking for.
All the basic mods such as uncensor, translation, cheats, unlocked sliders, and some other modifiers already exist and are not really hard to implement. Mods for custom clothes, hairs or accessories also exist and are not hard to create beyond the work in the editor, and that is something every modder needs to do no matter the game so in that sense it doesn't add that much extra work.
The main big difference compared to Koikatsu for example is when creating the big mods that change the game to a fundamental level. Those are the ones that you won't see in this game unless someones really wants to get their hands dirty.
However, if you think of Koikatsu, these kind of mods mostly exist for studio (like the timeline for example) while the game itself has almost no mods since the gameplay was so trashy (except for maybe kplug, and a lot of the community hates that mod due to how buggy and incompatible it is with other mods). If we think about AA2 the same happens, most mods weren't about changes in gameplay but about actually making the game work and adding QoL since vanilla AA2 was programmed like ass.
At the end of AA2 we got AAU which was kind of a miracle and allowed to really push the mods to a different level, adding things like the modules, but even then it doesn't really change the gameplay that much and most modules are buggy, so a lot of users avoid them.
In summary, what I'm saying is that I feel like most people actually overestimate the mods for games like Koikatsu or AA2 when in reality for normal gameplay they don't do that much new, and where the scene really shines is creating unique cards, clothes, accessories and stuff like that, which are already possible in HC and will be in SVS, so those are not the problem and it only depends if people is actually looking to create them.
The main difference will comes from studio/craft, where it will be almost impossible to reach Koikatsu levels unless someone really gets to work on it; and also for creating new personalities or adding new voices like in AA2, since I feel like creating those will not be easy for SVS. Beyond these, there's only a very short list of mods left that HC (and most likely SVS when it comes out) are lacking, such as better penetration.
So while a lot of people talk about KK and AA2 being heavily modded, they forget that most of these mods were made so the game would work, add QoL, and creating your own waifus and husbandos; things that HC already has. The question is if the other mods are really important for you or not, and I think that unless you really like studio or want to be able to customize entire body parts (like how people change entire heads in KK) of your characters, you should be fine in SVS.