In line with the suggestions by Ausman, have you also tried opering the Modbus with just the power meter and UniStream together, no other devices?
I haven't really seen any issues with other devices not running the same Modbus standard as Unitronics. One thing I do notice is that different vendors use different notation for the wiring - eg A,B, D+, D- . I have seen different vendors use opposite notation. It's easiest to sort this out with two devices at a time, and sometimes the solution is as simple as swpaping the wires.
Also when you get drops, what are the error codes telling you? Are they timeouts (no comms), or is the power meter communicating, but returning an error code?
My experience with power meters (Crompton, see below) is that they use 32-bit floating point values, but the Modbus address space works on 16-bit words. Hence only every second Modbus address is valid. Also beware of the +/- 1 offset. Unitronics calls the first register as 0, whilst some slave devices call their first register as 0 and others call it 1. Also if you a trying to read only 1 value, you will need to read two consecutive modbus registers. If the power meter is using 32-bit values, trying to read only a single 16-bit value is likely to generate an error.
I end up doing a fair bit of experimentation to find the right combination.
Disclosure: My company is a Unitronics distributor and also a distributor of Crompton power meters. I have set up several demos with UniStream to a Crompton meter and got them working on both Modbus RTU and Modbus TCP. I have also assisted customers with setting up many different Modbus devices on Unitronics PLCs.
So I could say Crompton meters work fine. However If you can't get 3 other brands to work, then I would be looking at the setup rather than the brand of meter.