They aren't categorically unsafe. They have passed the required crash tests and there is nothing that's less safe about them than any other forward facing seat.
The grey area where you might want to consider is the fact that their baby insert for rear facing is limited as it only goes up to 82cm, which is only likely to get you up to about 18 months rear facing. And while they say that a larger rear facing element is in the works, they have been saying that for ages and it has not yet appeared. So you cannot do extended rear facing (ie, RF past the baby stage) in this seat, and that is where it's true that there is a safety deficit because it is true that extended rear facing is safer than forward facing.
If your children tend to be on the smaller side and/or you'd forward face at ~18 months anyway, plus your older children are already forward facing, then it can be a reasonable option, though I'd consider the ages/sizes of the older children because the harnessed seats go up to 135cm/36kg, if the eldest child gets to this point before the youngest is old enough to be out of a harness, you may get an issue again.
Multimac actually did a video on the safety/development of the crash testing of it on their youtube channel, which is interesting and worth a watch.
Interestingly this video includes a crash test of the rear facing element with the 3yo dummy in it - so I don't understand why they have limited it to 82cm height, aside from the fact the 3yo dummy didn't have any leg room, but it does seem the founder does not see anything wrong with forward facing from 15 months, so it might be that they have set the limit for that reason.
Like anything car seat related it really depends what you're comparing against. If you're comparing against three children in rear facing seats, then the Multimac is not going to come out as safer. It might be worth investigating whether ERF seats with tethers (like Axkid Minikid), which can tesselate well with each other, other ERF tethered seats like Britax Max Safe, a high backed booster or smaller seats like Graco Extend R129, an infant carrier with belt fitted base, and/or putting one child in the front - whether any of this would solve your three across problem. I would look at an ERF specialist if you have one nearby or just a big John Lewis and see if you can play around with fitting different seat models next to each other, before you write off 3 across based on google because it's not so much about how wide all the seats are, it's about the shape of them and the placement of the parts which stick out basically. If you believe everything that is written online about getting 3 across you'd think it's impossible whereas I do think it's more often possible than it's made out to be.
OTOH if you're comparing against three very cheap, basic forward facing seats (because these are often the slimmest) or even two basic FF seats and one child in the middle with no seat, which is legal, then Multimac is almost definitely the safer option.
Of course, IRL you will have a multitude of options between 3x plus tested ERF seats, and 2x basic seats with one child with no seat - these are extremes, and Multimac, just as most setups, is somewhere between the two extremes.
Lastly since you asked about cars which fit three across, this list is a bit old but might be of some use: https://erfmission.com/car-seats-3-across-in-the-backseat/
Sorry for no easy answer there. But hopefully there is some helpful information somewhere.