My 2c. I've had a Fitbit (Charge HR) on my wrist since last May. I've found it quite useful, both for cardio (elliptical) and weights (free and machine).
While wearable trackers were essentially pedometers when they first came out, they've grown into much more sophisticated devices that do more than just 'measure steps'. There are quite a few now that also recognise weight workout routines. And can let you add routines it doesn't know about. Here's a couple of links to help you: https://www.wareable.com/fitness-tra...group-test-960 Best Fitness Tracker For Weight Lifting - Activity Tracker World
As far as 'bulk', The only time it kinda gets snagged is when i'm doing sideways pulls on the cable crossover machine, when the cable lays across my wrist, but isn't a major issue.
As to HR measurement, i've checked it against the measurements taken on the machines, and it seems to always be about the same as the machines reading. Now, the machines may also not be the most accurate either, but it does always pretty much match up...
There's also correct placement, for my Fitbit, it's one/two finger widths above your wrist bone (the sticky out lumps)
There was a lawsuit against Fitbit last year, claiming inaccurate HR readings: Fitbit Lawsuit: 4 Things to Know
Though, IMO, the 'testing' done by the plaintiffs was... halfa££ed. It takes a while for the device to get used to you, which wasn't tried in the testing. Put it on, work out for a short time, take it off, and wonder why it didn't 'know' you in the short period.
And the claim is that they (trackers) don't measure HR as accurately as a dedicated medical grade device. Well, DUH! Trackers are a multi function, have to do it all, for around $100 device. But none of the manufacturers have ever claimed (afaik) that their devices are as accurate as a dedicated medical grade device.