I am sad that this question is down-voted. It seems honest enough, it's slightly off-topic but not dramatically so.
There are two halves to the first answer but they're both "Yes, Let's Encrypt is just as secure".
1. Most elements of TLS security have nothing whatsoever to do with certificates. This is easier to grasp in TLS 1.3 than earlier versions (all the encryption in TLS 1.3 is working before anybody sends any certificates anywhere) but it has always been true.
Even without certificates eavesdroppers can't see what was communicated, and nobody can change it en route between client and server. For these things even no certificate at all would be fine...
Certificates do add a vital thing though: Identity. A certificate from Let's Encrypt is a signed document from Let's Encrypt vouching for the identity of your site. Cryptography (with a "private key") lets you prove this certificate belongs to you and nobody else can do that. Without Identity somebody in a position to be an eavesdropper could just pretend to be you and intercept everything (a "Man in the Middle"). So even though it's a small aspect it's vital.
2. To go around issuing people with Certificates you need some way to know who is who. Until a few years ago there weren't many hard and fast rules about how to do this, and so a lot of rather dubious procedures were used by people who charged a pretty penny. Some of them would argue that charging validated the purchaser but that's not so smart, plenty of crooks are willing to spend money to make more.
So, Let's Encrypt actually helped write actual formal rules for how you can make sure you're issuing certificates to the real owners of the names they're certificates for. These are known as the Ten Blessed Methods, because there were once exactly ten of them and each is a method that the certificate issuer is allowed to use to do this Domain Validation. None of them are utterly foolproof, and there is ongoing work to further improve them or get rid of the least effective ones, but at least now there are written rules.
Having helped write these rules it should be no surprise that though they represent a significant tightening up of things for some of the incumbent for-profit issuers, Let's Encrypt was already doing everything required.
Partly this is actually helped by not taking money for certificates. Since Let's Encrypt doesn't make a profit from giving you a certificate, they've no incentive to do so unless they're sure.
Now: For the second part, I have written lengthy answers elsewhere, there are a lot of reasons you might pay somebody money. None of these reasons make Let's Encrypt any worse, and many of them are real niche cases, you'd know about it if you've hit those. Like if you make web sites for Nintendo's obsolete WiiU video game console - Let's Encrypt doesn't help you because the WiiU web browser doesn't have the right trust store for that. Or if you need S/MIME certificates for your corporate email system for some reason, Let's Encrypt don't offer that. If you need a special relationship with your issuer under contract (like Facebook has) then Let's Encrypt can't help you. And so on. For most people it doesn't matter.