OK, so the first thing I would say is that there are two types of car seat company - those who make, design, research and produce their own seats from the ground up and market them under their own name, vs those who mass-produce generic seats not to sell directly to the public, but to sell to other businesses who put their own branding and logos on the seats and then market and sell them, all the same seat underneath but sold under different names for different markets.
The companies that make their own seats tend to put a lot of time and energy into development and as a consequence their seats tend to perform well and have high safety values.
The companies that produce business to business of course still need to adhere to the same basic safety regulations, but because of costs they do not tend to go much further than the basic requirements. Sometimes more premium models might have the higher end safety features in them. It's rare that these kinds of seats get picked up for independent testing (such as by Which?) but they do not usually do very well when they are. The more basic models in particular can do very poorly. It can also be difficult to identify a seat if it's tested under one name but sold under many others, especially as the tests are done in a central lab in Europe, so you often get the German or Italian version of these generic seats.
Kinderkraft is an example of a rebrander - they buy these generic seats and put their own fabrics and logos on and they market them like that. To be perfectly honest, this is a very flimsy model and I would be reluctant to recommend it. It may work OK as a high backed booster, where the job of the seat is purely to route the belt safely around the child, but I'm not sure I would want to use it in the harnessed mode, personally.
Also, if it is for a child who is just coming out of the infant carrier I would not normally recommend any 123 seat for them anyway, as they tend not to protect them as well as a singular Group 1 seat or even better, a rear facing seat.
But I realise this can be tricky if you're on a tight budget as well as tight space wise. Which other seats have you already got and what are the ages/weights of the children you need to accommodate?