Training using heart rate can be a good way to monitor exertion and control the intensity, however, it is important to know the limitations of heart rate training.
The most important thing, heart rate based training is only really useful for LISS.
Heart rate monitors often do not respond quickly to heart rate changes and may take some time to converge on the correct value. Also, quick sudden movements (for example lifting weights) may shift the monitor and skew the measurements.
It can have some use during HIIT, but how useful it really depends on the quality and capabilities of your heart rate monitor and more than likely would just end up as a tool to review your training.
Here are some recommended heart rate monitors to use:
Finally, many heart rate monitors simply suck. Most wrist-based heart rate monitors are going to have some error and if you want to train via heart rate, get a chest strap. I use my Apple Watch to monitor my heart rate, and it works fine for me.
Aside from the tracker, things like the weather/temperature, stress, how much caffeine you had, smoking, and a million other factors can affect your heart rate during training. Here is a good article on what effects your heart rate.
It is important that you have a good idea what your heart rate normally is at different paces and how it changes due to common external factors. This means if you are new to cardio, heart rate training is not going to be all that useful to you.