I drive in an arch mouth baucher cheek bit that is also a smaller diameter to better fit in the horses mouth. The arch mouth style mouth piece makes it VERY hard for them to get their tongue over the bit. My mare, Sky, HATES tongue pressure so was very insistent about putting her tongue over the bit. She is 16 and has been been driving since she was 2 1/2 years old, always in a roller mouth mullen style Myler half cheek. But for some reason last year she decided she didn't like that bit any more. Once I switched her, she tried to put her tongue over once and has been very content ever since! All three of my ponies drive in this style of bit and seem very happy.
Zorro drove 120 minutes yesterday with the bike. We were both covered in mud and happy when we got home! LOL!