It's not completely unsafe. It's misleading to say that it is (not you, but whoever told you this). Forward facing with a harness is safer than using a seatbelt if the child is under 4. A properly fitted 5 point harness actually does a really good job of protecting children. If it did nothing at all, car seats would never have caught on, since only Sweden started with RF seats (inspired by astronauts, apparently). Every other country inventing child car seats independently started with forward facing ones, mostly ideas which would be looked at as dangerous death traps today but they worked - they saved lives. What we have now is much better, but what we started with is still worth something.
The idea that it's dangerous or increases risk only applies to very young babies. In fact it's not even true to say that it's "completely unsafe" for a baby under 15 months old, though I would not recommend it, and the risk is unacceptably high given the alternatives, which is why it's illegal under the current standard.
Even for an under 15mo, it's much safer than being unrestrained or using a seatbelt. It's a dangerous myth actually - I once saw a question on one of those groups where the poster was in a low income country where car seats are not law, and only had the option to use a forward facing car seat for their 6 month old and was so worried about this that they opted to hold the baby on their lap instead. They had the risk the wrong way around - it is much safer to have the baby restrained than not, even though a 6 month old should definitely not be FF if at all possible.
The idea that it's risky for older children only makes sense in relation to rear facing, because rear facing is safer, although it was also put about by the companies promoting impact shield seats during the early 2010s. There is NO research study showing that seatbelts are safer than 5 point harness, for any age. It's just a myth, basically. There is a theory behind it which isn't proven.
The main reason RF advocates (including Swedish traffic authorities) have for discouraging use of FF harness seats is that they encourage/normalise FF before age 4. If your only option to FF is a high back booster using adult seatbelt and you know this is unnaceptably risky to use before age 4, then you're more likely to stick with a RF seat until then. They state it more like until 4 you can RF and then move straight to a HBB with seatbelt, so the FF harness seat is an unnecessary step, which is a completely valid argument, but it's not at all the same thing as saying that a FF seat is unsafe. They just mean it falls under the line that they would consider to be safe enough (and they only consider RF to be safe enough).
BTW, when someone writes a long post you can always tag them instead of quoting the post 😅 I always feel bad when people quote me when I have done my usual brain dump.