The main drawbacks, for me, are that the compressor is running continuously and also the chance of the airflow pulsing. With the tank, the compressor fills the tank and then turns itself off. You use the air in the tank and when it gets down to a certain pressure, the compressor starts up again and fills the tank, repeat ad infinitum. What I found when I started out with my original compressor;
was that I was still getting the compressor to run all the time as I was using so much air as I was doing so much spraying. That compressor still sits under my bench though, looking a little sad! I upgraded to a 25 litre tank compressor (from Aldi, funnily enough), which is still piston fed, and will be updating again soon to a belt driven compressor to cut down on the noise a bit more, as I still jump when it kicks in!
The other issue, pulsing, is more evident when you are doing fine work, which I have to say is not really the type of work you will do when spraying diecast models, it's more block colour spraying. The tank smooths out the airflow, so you're not reliant on the compressor providing the direct air. If you think about how the compressor works, the piston going back and forward will stop twice every revolution, which means a very very very small moment when there is no air being pushed out. In normal every day work you may not notice it, but it will be there.
It does all depend on whether you see yourself enjoying airbrushing and continuing to persevere with it. The trouble is that if you buy the Lidl kit, and struggle to get good results with it, you may be experiencing poor kit rather than poor technique, but still get annoyed and feel like giving up. That would be a shame as airbrushing opens up a whole new world of opportunities and quality of work. My advice would be to spend a bit more, get an AB or AS series airbrush and compressor with tank (and water trap and pressure valve) and really go for it. If you don't get on with it or have troubles, I am always willing to give advice as best I can, but if you really don't jell with airbrushing, then I am sure that you can resell the kit on far easier and lose less than the outlay for the Lidl kit. I'd probably even give you an offer for the airbrush myself.
It really is a case of you get what you pay for, in this case. Yes, there is a case to say "I'll use it to see how I get on and then get something better" but as I say above, you will easily sell a better kit for not much less than you paid for it and if you do enjoy the airbrushing experience then you will quickly get to the limits of the Lidl kit and want to upgrade, meaning your £40 outlay will sit there doing nothing.
However, I'm so sure that you'll enjoy airbrushing and I am always keen to promote people learning new skills, so I will throw an offer at you. As I say, I still have the compressor I started with in my workshop, and I still actually have the box in the loft. You are welcome to that compressor (just pay the postage), along with an airline and an airbrush (I have to check on that bit tonight) and if you get on with it, then we can sort out a price to buy. If you don't get on with it, just send it back to me. My advice is free! How does that sound?