I don't know. It's misinformation, TBH. I think they probably believe it, my impression of the car seat FB groups though is that a lot of people on there don't understand how to read research studies or differentiate good information from bad, and they often have a very skewed idea of probability. There are a lot of supposedly accepted truths which circle around and around and around those groups and nobody can remember who originally suggested it or what the context was, but often it's exaggerated, or someone made an assumption, or it was a misunderstanding that was never corrected, etc. There's also a culture of needing to present a united view in order not to confuse people and so that the group has a "clear line", and because of that, it all becomes very rigid and black and white, and real life (especially statistics and risk) isn't like that. It is part of why I left the groups because I felt like I was supporting misinformation and I also started to feel that the culture within the groups was toxic.
But in any case, there's really no evidence supporting the idea that FF with a 5-point harness is unsafe. In fact, as long as it's used correctly, it works really really well compared with the child being unrestrained, a seatbelt for children under about 4, or other kinds of FF restraint which are no longer used, like the American seats which used to have a sort of T-bar that came over the head.
It is true that rear facing supports and protects the spine much better than forward facing, and this is especially critical for very young children, which I assume you already know. By 4 years old, the chances of being injured because a child is FF when they could have been RF are much lower. They still exist, but it's a much smaller difference. You've been RF longer than probably 99% of parents in all developed countries, which is a really fantastic thing to have done. Even Swedish parents as a whole flip from majority RF to majority FF by 3 years, 9 months, according to an observational study by Besafe (in 2010).
There is a certain logic, which I understand some of the car seat group mods/admins subscribe to, and I can understand this view even if I disagree - they say that basically for each stage of car seats, you have a choice between various options, and why would you ever choose a FF harness seat when ERF seats exist. They generally say the same for why ever use a backless booster, when high back boosters exist.
I just think there are situations where other things, such as practicality, comfort, cost, can override - they do not agree, and this is fine. We can disagree! 
Some of the info about how young children have big heavy heads and FF seats pin back the shoulders, leading to an injury pattern of neck injuries in high speed crashes with FF babies and toddlers does come from Swedish studies about the benefits of continued RF after the infant carrier stage. However, none of the Swedish studies, not the original ones, and not the recent ones either, have ever differentiated between high back booster and 5 point harness. In fact hardly any European studies do. The studies which do this are mainly from the US and Australia.
I read somewhere that a company trying to promote impact shield seats was the main source of the idea (in online circles) that 5 point harness FF is inherently unsafe, or less safe than seatbelt use. I don't know exactly which company this was, though I could make an educated guess. But again, no studies have ever produced the evidence which would back this up, particularly when comparing 5 point harness with properly fitting HBB. In fact, for children under 4 and children under 40lbs (~18kg) there is evidence, both from crash testing and real world crash data, that 5 point harness is clearly better. After this point, which is roughly aligned, so we don't know if it's age or size that matters - it seems really difficult to prove much one way or the other, so it basically comes down to maturity of the child - ie, will they properly stay in a seatbelt - vs chance of misuse - a 5 point harness needs adjusting to the body size of the child and tightening properly each time it is used, can easily become twisted, relies on the seat itself being in good working condition, properly attached to the car and not too old, etc. So at some point, HBB wins out just because it is easier to use and the safety comes from the seatbelt, so far less chance of anything being wrong. There is also some danger to younger children from the fact that a seatbelt is much thicker and more rigid than car seat straps vs their bodies being relatively much smaller.
Neck injuries make up a small proportion of all fatal (or serious injuries) in all car accidents ever, even for children. Head injuries are the significant majority. RF advocates focus on neck injuries, because this is the major difference between RF and FF, but FF seats also work to reduce risk of serious injury and death overall. If they didn't work, Sweden would have been the only country ever to develop car seats because every other country started off with some version of a FF seat and discovered at some point in the 70s or 80s that they are pretty effective, and have been refining from that point on. Claiming that a FF child "will break their neck" in an accident is just not accurate, it is scaremongering nonsense at worst, or a complete misunderstanding of probability at best. The vast majority of accidents are minor and are unlikely to result in any injury at all. For a properly restrained child, this goes even for more serious accidents. The chances of neck injuries are small, and can be reduced further by RF. The chances of head injuries are high for unrestrained children, much reduced by any restraint at all.
If I ever want to make myself feel better about online car seat groups I go back and read one of the original 1980s US studies where there are some truly MacGyver style car seat set ups and the children - mostly - who were restrained, however bizarrely, survived. Then I think about how much better cars are now, how much better our seats are now, how much better our roads are now, how fatality stats keep falling even though not everybody uses RF. RF is worth knowing about, but it is not everything, and suggesting that it is, to the point of ignoring evidence or actively scaremongering about different types of FF restraint is unhelpful, IMO.
If you're going for I-irvana Max, which is fastened with isofix and top tether, the safety is likely to be very good, and if your child is wriggly and only just 15kg, I think it's the right choice - it would be my preferred choice too, I think. I have only just this past summer let my wriggly 5yo come out of the harness and that was because he is now too tall.