Closeness to the door is going to vary depending on the car anyway. We have DS3 in a Joie spin seat that has the little pop out bits on the sides. In our normal 5 seater cars, it is usually some distance from the door, but we recently rented a 7 seater Berlingo and on that you could only just physically open the pop out, as it would be almost touching the door. In some car seats/cars those things can't be used as the seat is too close to the door. Yes it's true you're going to be closer to the door than a standard car seat, but that is not necessarily a big problem.
The side impact protection in the Multimac is not good compared with modern, especially r129 car seats because it's an older design now, but you can get some additional side impact protection by using the YVLA headrest. An old Which review criticised the lack of side impact protection once the YVLA was outgrown, which used to be at approximately age 6, so it is now allowed and recommended to use the YVLA right up to the end of using the seat (so up to approx age 12). I understand that you do have to buy these separately, which is not really ideal. What you have to put into context here is that at the time that it was reviewed by Which, which was in 2010, the most common way to secure children aged 6-10 in the car was with a backless booster; this also provides no side impact protection.
I'm not going to argue that it's the best thing ever because it's not really; it won't suit most people, there are practical issues with its usage and in terms of safety in particular, it's over 20 years old as a design, but to be fair to them, they did use the most up to date cutting edge features at that time that were seen in barely any other car seats. Some of the basic cheaper seats today still have not caught up even to that. It's true that nowadays, the latest safety features far outstrip the Multimac. Extended rear facing for example is especially lacking, and the side impact protection could be better, but it's basically equivalent to a high end basic/older model or a "premium" version of some of the generic brands. It is the only non-special-needs car seat that can harness forward facing over 25kg.
"Most unsafe thing" is being totally unrestrained, or having a car seat fitted incorrectly. Most unsafe legal thing is probably some cheap basic car seat that a child is only just at the minimum weight limit for. Multimac is nowhere near either of those options. It can provide a solution in the case that for example the only other option is to have the third child with no car seat at all in the middle seat (which is legal, if you can't physically fit one in). Plus, it was originally designed to solve the problem that the boot seats in 7-seaters used to be pretty lethal in a rear shunt; I've no idea if that is still a problem in modern cars that needs solving, but it certainly does solve for that problem.