I fear that the outcome of metaphysical thinking will be needless suffering.
I don't want to suggest that all religious people will cause suffering, but that this approach to life is intrinsically blinding and unreasonable.
Here's how I think metaphysical thinking works:
1) Arbitrarily assume metaphysics M. (Or read about someone else's assumptions in an ancient book.)
2) M is consistent with every possible experience. It's not falsifiable because it's metaphysical.
3) Attach to M the imperatives Y, e.g., Pray at specific times, don't eat shellfish, sacrifice a virgin, accumulate wealth, don't accumulate wealth, keep slaves, whatever.
Now, observe that the imperatives Y don't have to lead to any particular outcome. Every outcome is consistent with M. So any Y can be "justified" by M, even though M and Y are completely arbitrary. If sacrificing virgins causes pain and suffering, you can claim it's God's will, or that God somehow balances things out in inherently unobservable ways.
I find this to be wholly unreasonable because any behavior can be justified if you admit metaphysical thinking. A "nice" metaphysician may reject "bad" imperatives, but he cannot criticize the bad imperatives of other faiths when they use the very same derivational methods that the metaphysician used himself (i.e., blind assumption).
To render a religion reasonable, you have to require that 1) your beliefs can be falsified (i.e., M is no longer metaphysical), or 2) that there are no imperatives founded on metaphysical claims.
Fortunately, many people are unwilling to accept Y or M if the outcomes are inconsistent with what they subjectively consider to be "good" outcomes. So, for example, the vast majority of people around the world will reject militant Islam (as metaphysics) or terrorist jihad (as imperative) because they regard the acts and their outcomes to be objectionable. This is what I mean by morality determining faith, and not vice versa.
Are there any religions that might be, how shall I put it... harmless?
Well, if the religion had no imperatives, that would probably be harmless entertainment.
Another possibility is if M merely specified what outcomes were classified as desirable. In this case, science and technology would be the tools used to make those outcomes happen. I find this idea suspect, but less so than any mainstream religion.
To summarize, the main problem I have with religion is that it isolates imperatives from outcomes, by tying imperatives to metaphysics instead. I consider action without concern for outcome to be irrational.