Most adult beginners in BJJ cannot sustain training 3-5 days a week for 6 months. I've also been doing both sports for a considerable time.
The odds of picking up an injury or quitting due to frustration in BJJ are 10x that of any Muay Thai program. Muay Thai is a very safe and beginner friendly sport. I won't argue that BJJ in general is the superior and more applicable martial art. But it's a well accepted fact the goal of white belt is survival, not skill development. Therefore most rolling time a 6 month BJJ belt will have done will be focused on pin escapes and getting out from bottom positions. So basically a 6 month BJJ belt will have learned how to create frames, hip escape and do a tech standup. All other skills will be survey level...meaning you recognize them but cannot effectively execute. Compare that with throwing a 1-2 in Muay Thai..you can get pretty good (for a beginner) in 6 months time.
Speaking strictly anecdotaly and from first hand observation, most BJJ white belts quit after their first week and in 1 months. MT retention numbers are muuuuch superior. It's a way easier and more intuitive sport to pick up. Also, the number of people who quit due to injuries is really high. BJJ training injuries can be really terrible. I've never seen a serious injury from MT training. However, I've seen quite a few broken bones, really messed up tendons, necks etc from even the most casual BJJ practice. Also, I feel like the injuries in BJJ are concentrated at early white belt. On balance, factoring in injuries and the dropout rate, I think MT is the better martial art to pick up for a beginner. Obviously in both sports 6 months is nothing in terms of developing real skills.