I was witness to, though not directly involved in, just such an attempt a week ago. The owner of the 6 Plus is now in the market for a new phone.
Let’s just say I’m not convinced the process is reliable. And if you pay a professional to do the work, you’ll likely be spending a significant percentage of the cost of a replacement device anyhow. 7s and 8s are pretty darned cheap nowadays. A quick search brings up refurb 7s for under $200.
I have changed several batteries so far, also for friends and family, and I never had any trouble, but I guess eventually I will break one, too.
Of course, if you’re uncomfortable doing these things, don’t do it. The fault lies with Apple, they should be forced by law to make the battery simple to change.