They are not safe, IMO. The legal minimum testing is a 30mph frontal crash. They will pass these, when perfectly installed and with the harness done up properly. The problem with cheaper seats is that if you have any play in the harness or any small mistake in sitting they do not perform. Some of them also end up recalled after they fail more stringent independent tests (40mph including frontal, side impact and rollover) by organisations like Which? and ADAC.
If I knew that a child was going to be using this seat for journeys around town and I could be confident that they would be well installed and the harness used well then I'd be okay with it. My childminder used to have one and she only used it for nursery pick up on rainy days. The problem is that another issue with cheap seats is that the fitting and instructions are often confusing and difficult to get just right, which increases the chance of incorrect usage.
They aren't made in China BTW, they are made in France - not that that really makes any difference TBH! Teamtex is the company behind Nania and they also produce a lot of the other cheaper seats on the market. They all use one mould so you can easily check if it's a teamtex seat by comparing with Nania models.
Two companies I'd trust for cheap seats are Joie and Britax. Unlike others on here although I do agree that continued rear facing is safest, I don't think it's absolutely necessary and in some cases it really can be impractical, so my take is more to rear face for as long as it is reasonably practical to do so. The main danger from forward facing occurs at up to and around a year of age and it's to do with development and head size rather than height. So I'd always try to rear face until 15 months if possible as this avoids the most risky period. This is where the newer i-size regulations are coming from - it's based on evidence that before 15 months is the most risky period for FF related injuries. Of course, the risk doesn't disappear at 15 months but it is lower. So I'd prefer to RF for 2 years but where I disagree with others is that I don't feel it's strictly necessary for safety at this point. For you with a 14mo it's borderline for me, but practicality is important as well. RF is always safer but whether a good FF seat is safe enough is a different question. I certainly would not look at the ultra cheap FF seats, but FF past that "around a year" hump in a budget end Joie or Britax (or better) is okay with me.
Legs are no problem - children tend to find different seating positions to adults comfortable. They actually often like being able to rest their legs on the back seat or cross them or fold them up, rather than having them dangling over the edge of a FF seat with no choice in where to put them. Once my son was in a booster this has been a comfort issue for him on long journeys which we had to solve by putting a suitcase under his feet for a resting place! If you're worried about your seats getting muddy just remove his shoes in the car, which he might also find more comfortable.