TBH I would be sceptical of the claim that the car seat is "no good" - Red Kite is a reasonable brand even though they are on the budget end of the market, their car seats conform to the same safety standards as any other on sale in the UK.
What I suspect people mean is either that they want you to buy a car seat from a specific brand - safety wise, there is no reason to do this, as long as your car seat is approved to R44 or R129 and is suitable for your baby's age and size, it's fine (and this one is R129, the newer regulation).
Or they mean that car seats from budget or fashion focused brands are sometimes not designed well in terms of newborn position, comfort and ease of use. This one is more likely to have truth behind it, but whether it matters enough to replace the seat is more of a personal decision.
Not all travel systems do have the ability to change the car seat, although the more manufacturers move to generic producers the more they all rely on the same systems. I can't find a picture of the car seat adapters for this pushchair, but if you want to trawl through video reviews, you will eventually normally find a shot of someone using them, or of course if you already have it, you could look at the ones it came with.
If you google image search the phrase "Maxi Cosi car seat adapters", you'll see a selection of different car seat adapters that all have the same shape at the top - it's a sort of cut-out U shape. If your car seat adapters look like this, you can put a different brand on them. If they look like a different shape, then it's probably only Red Kite car seats which will fit.