Getting up from prone (in RAW d&d and BG3) costs half your movement. Prone also gives you advantage on melee attacks to targets that are prone (this is the advantage). The actual problem with prone, that I see, is that it ends your turn (a DoS engine side effect). (For example: when your walking over an ice surface on your turn, walk 5 feet and slip, you lose your turn, your action, BA, and 25 feet of movement you could've used to get up)
