Hmm, yes and no.
If for example you bought the Joie 360 version which spins 360 degrees, there's nothing that different about that seat compared to the spin safe version that only spins to the side. So it would be just as safe, as it's essentially the same model.
For a completely different seat though you can't make the same comparisons. And unfortunately it seems like all the ones that seatbelt fit are made by generic rebranders or budget manufacturers. As a car seat is a safety item, I wouldn't be comfortable choosing a budget item, particularly one with fancier features like a spin. That spin mechanism is likely to be a weak point in a crash, so if choosing a cheaper model I'd want the reassurance that it's been through extra testing like the Joie ones have.
In addition a problem with belt fitted seats which rear face up to 18kg is that the belt usually comes around the back of the seat to give support if you had a larger child in the seat. And if the seatbelt is here, then you can't spin the seat. Some of the cheaper spin seats do work this way, and it isn't really advertised. You can only spin it to easily turn it from rear facing to forward facing.
If a seatbelt fitted car seat can spin and doesn't use this belt path, usually the seatbelt is only routed through the frame or base of the seat, rather than around the seat itself. That means that it wouldn't safely support an older child so these seats tend to have lower rear facing limits like 13kg or 10kg.
Also in addition even for the isofix ones, when some of these cheaper spin seats have been tested by consumer organisations, they have found that the seat moves a lot when rear facing and the child could be in danger of hitting the front seat, which would be dangerous. Rear facing is very safe, but only if the seat is secure.
What you could consider instead of a spinning seat is a rear facing seat which goes up to 25kg instead. These are all belt fitted because of the higher weight limit but they use tethers as well so they're really secure. They don't spin but the sides are nice and low, so you can get the child in and out easily. And a lot of them are plus tested.
The only belt fitted spin seat I would be comfortable recommending is maxi cosi Axiss, but that's only forward facing, so definitely won't be as safe as a rear facing seat.