AFAIK, the only reason not to use Letsencrypt are when you are not able to automate the process to change the certificate.
As the paid certificates are valid for 12 month, you have to change them less often than a letsencrypt certificate.
At work, we pay something like 30-50€ for a certificate for a year. As changing certificates costs, it is more economical to buy a certificate.
But generally, it is best to use letsencrypt when you can automate the process (e.g. with nginx).
As for the question of trust: The process of issuing certificates is done in a way that the certificate authority never has access to your private key. You don’t trust the CA with anything (except your payment data maybe).