I love the flag story
My reasons for leaving Mahayana/Zen and Pure Land is because of my inability to justify some of the teachings with others I have talked to. I couldn't justify praying to Amitabha Buddha for rebirth in a land of bliss based on the idea that beings nowadays cannot attain Enlightenment by themselves. Every time I tried to differentiate Pure Land from Christianity, I always ended up coming back to Christianity. And during my time studying Korean Zen, I couldn't justify praying and bowing to countless Buddhas and Bodhisattvas, practicing karma-cleansing rituals and forgiveness sessions. Maybe I was doing it wrong, which is likely actually. But a pull towards the forest tradition and strong urges to study the teachings of various Ajahns ultimately lead me back to Theravada.
I kept telling myself that these practices are really just to cultivate mindfulness and do away with a concept of the self. But then I had to be real - you can practice mindfulness without praying and bowing to 88 Buddhas every Sunday. And save your knees a lot of suffering
I also found the concept of forgoing Nibbana until all sentient beings are saved a tad bit unrealistic, especially since the Buddha praised the attainment of Nibbana and Parinibbana and made it the ultimate goal. It's the cessation of suffering, which is what Buddha taught. Without Nibbana, what is the point of Buddhist practice other than hippy, lovey-dovey feel good? And it's funny, because Bodhidharma supposedly taught against ritual worship, and yet it seems Zen is full of it.
But I do find many Zen teachings to be incredibly profound and relevant to western life. The emphasis on emptiness has always been a cornerstone in my practice, even now. And I just love the work of Bodhidharma. Furthermore, ancient Japanese and Chinese culture has always been an interest of mine. But when I found out just how much of Zen is influenced by Taoism and Confucianism, that gave me more reason to embrace the forest tradition.
But I really do see Zen and Theravada slowly becoming more understanding and accepting of each other, especially among the laity but also even among monastics. I think some of the concepts found in Theravada are simply exaggerated and emphasized more in Zen (such as their being many past Buddhas and Bodhisattvas, emptiness, mindset at death determining rebirth [as in Pure Land, with the idea of dhamma-decline and enlightenment based on faith], etc). But ultimately the teachings point to the same moon
However, I must say that I find it to be a matter of interpretation, which is very very important in my opinion. I think Mahayana has a somewhat skewed view of what the Bodhisattva actually is and what they actually do, especially because most all Zen teachers and practitioners I have heard from believe Theravada has no Bodhisattva ideal at all, which is completely false. Furthermore, Zen seems so inconsistent depending on who you talk to. Some Zen and Pure Land practitioners believe the Pure Land is just a mind state, while others take it quite literally as a place you are reborn in. It seems that most Zen practitioners hold real, actual Buddha-nature to exist, but some others dismiss it as just another translation of a universal spirit or Being or energy or something like that.
I guess I must admit also that Zen seems, in my perception, more focused on the laity and it appeals to them more, especially with the idea of sudden Enlightenment. But I am very much wanting to live the life of a renunciate, and I relate more to teachings coming from the renunciate tradition.
When it comes down to it, I think I could argue the similarities between Zen and Theravada up until we get to sudden vs. gradual enlightenment and authenticity of various texts. At that point, real differences emerge.
I hope I made some sense here. Forgive if I didn't.