The good thing about “megachurches” is that they can gather together a pool of leaders with various gifts, and can do certain kinds of ministries that smaller churches can’t, particularly youth and young adult ministry. As the culture becomes more secularized, this is increasingly important.
The problem is non-denominationalism. Christians read scripture within the context of tradition. Even those who emphasize “Sola Scriptura” a lot, invariably look at how their predecessors interpreted the Bible. Christians who lived 200 years ago were not necessarily wiser than us, but they had different insights, and different blind spots from us.
You could go fishing in the wide ocean at random. But it helps furthermore if you can focus your research. For instance, Lutheranism is a consistent baseline, they had a consistent terminology and themes over time. The advantage of choosing any one tradition - say Presbyterianism - is that it provides a discipline, it structures you to consider things you might have avoided, or not found at all, if you did a random search of “tradition” in general.
Non denominational churches lack that discipline. They become “accountable” only to their people - their consumers. Consumer driven churches tend not to bring about conversion, they are like cafeterias where you choose what you feel you need.
Having a denomination of course is no guarantee of truth and goodness and conversion, as evidenced by the mainlines. But those churches all moved away from their denominational roots anyway, so they are more or less non denominational. They don’t benefit from the structure and discipline they formerly had.
