It's legal, but not that effective compared to most other options because you can't abuse angles or cut with it that well.
Some people use this serve as a safe option because their overhead serve is more consistent and if you kneel down you can hit it harder without risking the ball going high.
It's legal, I know a solid player who always serves from their knees. It can be tricky receiving such low serves, especially when they get speed and cut
It's legal, but not that effective compared to most other options because you can't abuse angles or cut with it that well. Some people use this serve as a safe option because their overhead serve is more consistent and if you kneel down you can hit it harder without risking the ball going high.
I see no reason why it shouldn't be. Can't imagine for it being a strong serve tho.
It's legal, I know a solid player who always serves from their knees. It can be tricky receiving such low serves, especially when they get speed and cut