isofix vs seatbelt -
Isofix improves safety in two ways:
It reduces user error - misfitted seats are implicated in the majority of serious injury/death in car crashes where children are in car seats, so it's a huge issue not to be taken lightly. There are many possible ways to err when fitting a child seat with seatbelt, and most of them are serious with the potential consequences being huge. Isofix is specifically designed to be foolproof, with red/green (and sometimes audible) indicators and easy steps - you basically just click it in and go, and if you get something wrong it's usually minor.
Something like half of all belt fitted seats are fitted incorrectly, whereas only about 20-30% of isofix seats are fitted incorrectly. The figures vary because they are always small sample sizes and it depends on what they are counting as misfitted.
The second thing is that Isofix seats have what's known as a third point of anchorage. Any car seat is "anchored" into place with two points - the isofix arms that you click in, or for a belt fitted seat, the two ends of the lap part of the belt are the first two anchors. The third anchor might be a support leg or a top tether. The purpose of this anchor is to prevent the seat from tipping forward in an accident, which helps reduce severity of injuries. So you do see measurable differences between e.g. a forward facing belt fitted seat like Maxi Cosi Tobi and the isofix version of the identical seat e.g. Maxi Cosi Tobifix. Or the Britax Duo for example, was the first isofix seat, and it was allowed to be fitted with and without the top tether, or with seatbelt - it performs better with isofix + top tether.
So for the first point, a belt fitted seat only performs worse than an isofix fitted seat if the belt fitted seat is fitted incorrectly, which is more likely than for an isofix seat. But for example for infant carriers when they are fitted with base (isofix) vs belted, they often perform almost exactly the same, even without the base is often slightly better. As long as you get it right, belt fitted is just as good.
So if you buy a Swedish type ERF seat like the Axkids, most people buy them from specialised vendors that only or largely sell ERF seats, and the seller will generally talk you through fitting and teach you how to do it properly, or in some cases do it for you. That isn't always possible if you buy it online so you definitely want to watch some videos, read the manual and basically ensure that it's definitely installed correctly without the ease of isofix.
And for the second point the ERF seats are quite unusual for belt fitted seats, in that in addition to the seatbelt they also have a support leg (like isofix seats) and they have tethers which go down and secure under the seat in front. All of this makes the seat very stable and secure, much more so than your average belt fitted car seat, and even more stable than some isofix fitted car seats.
So in this case, no it's not a problem that it's belt fitted, but you should make extra sure that you are fitting it correctly.