How safe are downloadable lock apps? I've noticed quite a few lock screen apps and some of them do seem to be rather interesting.

But, are they really that secure? I've seen some where I can bypass them by using the Recently Opened Apps list. I've also noticed that some of them show whatever was previously up when I turn my phone on again.

These shortcomings seem to imply that they're very easily circumvented. Is this true and is there a way to make them more secure?
It depends. Some are secure while most are insecure. For example there are finger print apps. Most of the people knows the trick so its insecure.