I have been using a couple of these apps for years and have never been nagged to upgrade or pay. I'm an f-droid user, so that's probably why.
A lot of FOSS apps are taking this approach: offer the full version in f-droid, charge for it in the play store, both to encourage f-droid use and pay some bills. There's nothing wrong with it.
The only problem I've ever had with it was I used the contacts app when it was new and it didn't actually fill in phone numbers on vcard export. This was terrible, thankfully I had an older backup and only lost a couple of contacts, but still. Beyond that, these tools work extremely well.
A lot of FOSS apps are taking this approach: offer the full version in f-droid, charge for it in the play store, both to encourage f-droid use and pay some bills. There's nothing wrong with it.
The only problem I've ever had with it was I used the contacts app when it was new and it didn't actually fill in phone numbers on vcard export. This was terrible, thankfully I had an older backup and only lost a couple of contacts, but still. Beyond that, these tools work extremely well.