Some of them aren't great, but you have to bear in mind that a lot of the criticism about Cosatto comes from rear facing proponents, many of whom would never recommend any forward facing seat ever.
As there is no independent crash test result for this particular seat, I can just try and give you some thoughts on the model in general.
The Judo is a lot more substantial than for example the Zoomi which doesn't look as though it would offer a huge amount of protection in a crash.
It has top tether which is great. Top tether used in conjunction with isofix can really help reduce forward motion in forward facing seats.
The Cosatto Hug did get a poor crash test result a few years back - this was a similar model ie a Group 123 seat which can either be belt fitted or fit with isofix + top tether. I've just looked up both to see what the differences are, and they are quite different seats. The Hug is on a plastic base which can mean there's a degree of separation/movement between the seat and the base. It also has quite a confusing belt fitting process and not much in the way of tensioning the belt. Belt fitted seats like this tend to do worse in crash testing, and that might be the problem with this seat. Unfortunately, the free version of the report doesn't differentiate between belt fit vs isofix fit and neither between 5 point harness mode vs high back booster mode. Possibly the paid Which? report does.
Whereas the Judo is a one piece seat. It uses what we call a serpentine belt path (over, under, over) which is really secure and one of the better options in terms of belt fitting a seat. In fact if you are looking at 123 seats in general I'd always recommend to get one with a serpentine path or isofix and top tether, so I'm happy with both fitting modes there.
It is very much a "booster with harness" type seat which is not necessarily a bad thing, but might mean for example that younger children under 2 years old don't fit very securely, because there is too much room around their hips to move around. I tend to prefer this kind of seat for children age 3+ who just need the 5 point harness a bit longer before moving to the seatbelt. But 2y3m is not the end of the world, if you think he fits OK in it. On the plus side this type of seat tends to have a pretty good belt fit once they do convert to a booster, whereas the Hug which is on a base, those don't, which may also have contributed to the poor crash test result on that seat.
In terms of changing seats - there is no sense in doing that unless you're changing to something safer. Whether you can do that on a budget I'm not sure.
The cheapest and most effective way to improve safety would be to change to a rear facing seat, if that's something you're open to doing. You can get rear facing seats up to 18kg for as little as £70 (e.g. Joie Tilt, Graco Extend). Or up to 25kg they start at £159 for Britax Two Way Elite. I wouldn't recommend using these seats forward facing - your current seat is likely to be better. But you could get an increase in safety by rear facing.
If you wanted to stick with forward facing, you might be able to improve safety by swapping to a seat that covers only Group 1, rather than both the harness and booster stage. These tend to cradle younger children a bit better and get better crash test results than 123 seats overall. For example, you could look at Maxi Cosi Tobifix for isofix or Maxi Cosi Tobi for belt fitted, but they are both around the £150 mark. The forward facing seats which are really top notch for safety are quite a bit more expensive - e.g. Maxi Cosi Axissfix Air.
As said I don't think you'll find much improvement with other 123 seats, it already seems to have everything I'd look for in one. And there's no sense in changing to something that performs roughly the same. Britax Advansafix is probably the best on the market of all the 123 seats, but it's around £180-220 depending on which model exactly you look at. There are the impact shield ones, which tend to get better crash test ratings and are a bit cheaper, but (to cut a long story short) the crash test ratings are not the full story on those and I'm not convinced they are in fact any safer, so again I wouldn't switch to one unless you need to.
If you choose to stick with your seat, the most important thing is to make sure you're using it as the instructions dictate, common errors are having the safety harness set too low or too high, putting children in the seat with winter coats on, safety harness too loose, or not using the top tether (when isofix fitting). If you're seatbelt fitting double check you are doing the correct order (over-under-over) as sometimes people do it the wrong way around, and it's a good idea to kneel your weight into the seat when you pull the seatbelt tight to get it as well fitted as possible. Lastly, put the front seat as far forward as possible/comfortable to minimise any risk of contact with the seat in an accident (it also means they can't kick you in the back!)