I have a professional Ultrasonic receiver, I rarely found it useful
A good receiver with a tuned magnetic loop when combined with a AM HT and yagi will find 99.99 % of interference.
Home made H field probes are very useful for pin pointing the source of RFI once the source has been located in the home
I live in a remote area and have found an electric bicycle really good for prowling around at night trying to locate RFI easily.
I can cover large areas faster than i can on foot. I can sweep past poles in minutes trying to pinpoint noises.
The real big issue these days are LED lights both garden and floodlight. The other sources of noise are solar inverters and the tuned magnetic loops do the best job tracking these down.
But any tool in your RFI warfare kit will be useful. I will pick up the MFJ unit for the club and give it a try.
Save your bucks, I have a ultrasonic device, (and it works great for what it is designed for), and have used it once in two years... Use the same bucks to get a good AM radio and loop...