So often it is presented as such a highly scientific technology that can only be created by “experts”. Buzz words like “scientific”, “certified engineer”, “frequencies”, “cortical evoked response”, “anterior brain activity”, are used to make it seem over-complicated, and pseudo-scientific pictures of “data” and “brain readings” are pasted all over sites to further qualify claims of how powerful and amazing these people’s product is.
Rudy's primary work is on Alzheimer's. In this disease, beta amyloid proteins form plaques in the brain and kill nerve cells eventually leading to dementia. Research from Washington University School of Medicine in St. Louis shows that when people's brains are in a Delta brain wave state, beta amyloid production in the brain ceases and the toxic material is cleared away. One thought is to explore the possibility of using brain entrainment technology to help treat Alzheimer's, but at this point all it is being used for is to help people meditate, relax and dream instantly and effortlessly.
One RCT (n=108) showed significant reduction in anxiety from a single session of alpha/delta therapy for day surgery patients. A crossover RCT of a single session of theta stimulation in four healthy adults reported significant improvement from the intervention in one of five measures. Five pre/post studies reported significant benefit from the intervention for 16 of 27 outcomes.
I can offer a hypothetical--however do not know know practical it will turn out to be. There is a fair amonut of bad science on this topic on you tube. The gamma wave frequency is associated with attentional focus and arguably a unified sense of self. If it could be entrained it should lead to a meditative focus-like exceptionally well-trained meditators. Some but not all studies on meditating Buddhist monks suggest this is so. Does this really and more generally occur--I will say there is nothing lost in trying--we all go into a 40Hz range on occasion without ill effect-- but I would not put confidence in the purported methods out there at present. Good luck.
Monaural beats have similarities to binaural beats, but the two separate frequencies are delivered through a pulse pattern and mixed before they reach the listeners ears, resulting in the advantage of being audible without stereo headphones. However, using stereo headphones intensifies the effectiveness of monaural beats and is recommended for the best effect. 
True on your point about diminishing returns. It is like dieting. The bulk of the weight comes off at the beginning and then you lose a pound or so if you are diligent. Yes you lose the high of the original gains but you don’t receive more gains because you are in a better place. I am a big believer in data. When the gains begin to be less obvious a daily entry of how your emotional state was (1-10) and naming something you are grateful for goes a long ways. I guarantee a person will overall be in a better place on average. As always, thanks for your contributions Daniel.

Subsequently, the term 'entrainment' has been used to describe a shared tendency of many physical and biological systems to synchronize their periodicity and rhythm through interaction. This tendency has been identified as specifically pertinent to the study of sound and music generally, and acoustic rhythms specifically. The most ubiquitous and familiar examples of neuromotor entrainment to acoustic stimuli is observable in spontaneous foot or finger tapping to the rhythmic beat of a song.