It really seems like it should be easier to reduce a list of Results

monads.rb

require 'dry/monads/all'

include Dry::Monads::Result::Mixin

success = Success(:finished)

def merge(result, failures)
  result
    .or { |failure| Failure(failure + failures) }
    .bind { Failure(failures) }
end

result =
  [success, success, Failure(2), success, Failure(4), Failure(5)]
  .reduce(success) do |acc, el|
    el
      .or { |*failures| merge(acc, failures) }
      .bind { acc }
  end

puts result  #=> Failure([2, 4, 5])