All I use are Daz V4 and M4 currently (in the past I used V3 and M3 reduced resolution, but now with C8Pro and since I've moved to a 64 bit computer there seems no reason not to use M4/V4 even for my unimportant 'background' characters).

I have no problems.  I'm not saying it didn't take me some time to figure out how to use them with no problems, but currently I have no problems (that I'm aware of) all morphs work correctly.

I do use Carrara magnets to help simplify clothing fits as I find they are usually simpler and faster than fiddling with umpteen-hundred different clothing morphs and experimenting to find the one that will make the clothes move in the direction I want).

I have no idea whether bone scaling has problems in Carrara; I'm not sure what that would refer to.  I used Daz Studio a total of once, and found it so counter-intuitive and difficult I never used it again, so I'm not familiar with what d-forms are or how they work.  Poser magnets I know how to use but in Poser it took me too long to use them so I generally avoided, again in Carrara I at first avoided magnets for the same reason (thinking they would just be too complex to figure out) but eventually I discovered they were easy enough to use.  So working in Carrara, I do use the Carrara magnets and haven't felt any special urge to try to import Poser magnets into Carrara to do just the same thing.

If you were going to use Poser stuff in Carrara the two things you have to learn/understand is how to retexture your Poser stuff once it's in Carrara so that it's not too shiny, and also how to import the Poser stuff so that any morphs come through intact.  Again, I have no idea why you would need Poser magnets in Carrara (maybe there's something I'm missing), and I'm not certain what bone-scaling and Studio d-forms are, so I can't speak to that.

But I have no issues (that I'm aware of) with using either V4 or M4.