4:1:1 is half of RGB but I've never seen anyone ever use it. Pretty much everything in use today (at the consumer level) uses 4:2:0. It's good enough for today's standards (AV1 Main profile only supports 4:0:0 and 4:2:0).
Assuming infinite precision, sure.
Interestingly however, there's a closely related color space called YCoCg, that actually does achieve lossless coversion to/from RGB (this lossless variant is often referred to as YCoCg-R, the R stands for "reversible"): https://en.wikipedia.org/wiki/YCoCg#The_lifting-based_YCoCg-...
YUV to RGB conversion (and vice-versa) requires use of conversion coefficients (BT.470, BT.709, etc): https://en.wikipedia.org/wiki/YUV
Different apps/algorithms can choose their own coefficients, so you get a slightly different RGB colors. If you converted RGB back to YUV with a different set of coefficients, you would get a different result.
The difference between 4:1:1 and 4:2:0 is that in 4:1:1 the chroma covers a 1x4 line of pixels whereas in 4:2:0 the chroma covers a 2x2 square of pixels.