For:

• more likely to achieve changes
• potentially faster changes
• tongue is not mighty enough to generate "violent" forces even at max strength

Against:

• your technique may fail you, especially in the beginning; you may be mewing wrong
• you may fail at realizing the connection between tongue posture and the rest of the postural musculature, potentially leading to injuries and/or unfavourable changes.
• fast skeletal/structural changes require fast postural & nervous system adaption, increasing the risk of strains and other injuries due to changed patterns of movement & body motorics.

Progress has covered important reasons. There is also the reason of "over correcting". When you try to push hard you end up engaging other muscles that should not, such as in the face and throat. This is a concept from physiotherapy that applies to all the muscles in the body and its a mistake that people fall into all the time. 

This leads to another problem. If you keep a muscle contracted for long periods, as in hard mewing, this will lead to straining and overtime develop trigger points. I don't think trigger point will develop on the tongue but certainly on the face.

If you are interested in more details I covered this topic in several posts:

/community/community/main-forum/if-you-are-doing-hard-mewing-you-will-regret-it/#post-10450

/community/community/main-forum/light-forces/#post-8291

/community/community/main-forum/hard-mewing-might-be-very-useful/#post-7537

/community/community/main-forum/hard-mewing-might-be-very-useful/#post-7556