The larger the capacitance, and/or the higher the test frequency, the lower will be the capacitor's impedance. Large caps at high test frequencies result in immeasurably small AC values across the Device Under Test (DUT) - that's what I am driving at. I didn't ever bother trying to test the 470uF above 1 KHz.
Summary - these ESR meters ARE DESIGNED to measure components in circuit. The onus is still on the user to understand enough about electronics in general, and the circuit under test in particular, to know if such measurements are indeed valid. Collections of closely wired networks of power inductors and filter caps in power supplies, or smaller components in complex close configurations in filters, will interfere with one another, when you try and take measurements on an individual component. No ESR meter can circumvent this!
I have a Mastech 5308, and it suits my Hobby/home purpose. I probably wouldn't specify it for use on a production line. I've no doubt the Agilent is even better - and so it should be! But I just have no need for any such Betterness. (I think I might have just made that word up).
Not a great fan of UNI-T equipment at all, but no experience of their LCR meter. If I have one criticism of my Mastech, it is that it comes in a larger case that is really at all necessary. The UNI-T's more "multimeter style/size" case might win some people over.
<EDIT> Does the UNI-T device use the small 9V ("PP3" if you are old enough...) batteries? Hmm...the chipsets in these ~$150 meters will burn through little batteries in double-quick time (22 mA draw with the backlight on). The MS5308 does use 8 AA batteries.