I have noticed that the Garmin optical is not reliable for me. It doesn't pick up hard efforts for a good 10-15 minutes (just guessing, but it's a while). But eventually gets close enough. But looking at the data on my girlfriends rides (I gave her my old watch), the data looks pretty good. So I am guessing something about my skin doesn't play well (thick skinned?). But I use that data on any rides where my HR isn't a concern. If I want accuracy, I use a strap.
That said, the few Garmin straps I have used since 2012 have been great. The most recent one seems to be the most reliable of all of them. I bought an HRM/strap combo instead of just a strap as the price difference was surprisingly low, and the new one has been working after a couple years of abuse.
I would say that if optical works for you, and you don't mind a watch, the Garmin watch's are very convenient. They do transmit data to your head unit, which is what I do on my road bike when I don't use a strap.