I've had a lot of experience with repairing and test running these MSS Locos.
I run the business called Model Enthusiasts and we have been selling them for some years now, as well as mending them.
To help you see them from my perspective it may help to look at them from two viewpoints.
1. Buying a kit or ready built loco
2. Repairing a loco.
Buying a kit or ready built loco
The choice is either a kit or a ready-to-run loco.
If you are happy to spend time assembling then it is a very helpful and interesting way to learn what makes it go. The written instructions are clear enough but of course, a leaning towards mechanics or engineering does help. Most of our "kit" customers are pretty practical and I think that is important. Mechanical intuition is generally all that is required to work out why something isn't fitting or is not work properly. That just about describes who should and who shouldn't buy a kit loco.
Ready to Run Locos:
Joy of joys. It's all been sorted out before it leaves the factory. Or has it?! Let be realistic about what you would be buying and the price. The first clue concerning "should I buy one?" is the price. They are the lowest price live garden rail locomotives available. So maybe they score well there. However, are they fast off the blocks when first fired up?
They are manufactured on a batch production basis. There is no conveyor belt production line. And as with any bench assembled product little slip-ups can occur during assembly. Occasionally little niggles do arise but nothing that can't be put right.
Your money buys you a simple, tried and tested garden railway locomotive that gets better the more you run it. Press on through any priming issues (water/steam from cylinders), be patient, maybe talk to your supplier for advice and soon you will have an engine that runs a treat. If you want to see how some owners get on with them, you can check out the YouTube videos. That's a great source for an unbiased opinion.
If you run the loco as it should be operated, that is with solid fuel, careful water level observation, and adequate application of the correct oil, you will have an engine that will give you trouble-free running.
However, in real-life, owners will drop the engine, run it with a gas burner and not check the water level and forget to oil it properly and so achieve an unintended but predictable outcome. But that's about human nature eh!
At the end of the day, these little fun engines are very strong, sturdy and repairable with all parts available as spares on-line. So the final analysis says that for pure raw pleasure at a reasonable price, you can't get a better bang for your buck if you like small live steam locos.
That's my opinion anyway.