Yes, urine can have a pretty high salt concentration, up to around 2% by weight. As a point of reference, I think seawater is around 3.5%, so urine is fairly salty. If you dilute it and use it sparingly, you shouldn't have a problem, since the salt will wash out of the soil over time. But if you overdo it, you can alter the salinity in your soil, and most plants really aren't fans of high salt levels. I imagine the amount that builds up in the soil will depend a lot on your soil type and how well it drains (and how frequently you get good soaking rains).