I had old SLX systems drop out in the middle of wedding ceremonies fairly early in my career, and I was willing to pay a premium to feel better about the solution even when working in the very RF heavy DC venues.
When using 2 or more mics at a gig the number one cause of dropouts is poor frequency coordination. The number 2 cause is poor antenna placement, or the wrong antenna choice.
Most DJs I've dealt with who experience this issue simply don't realize that ALL of the mics must be assigned to the
SAME GROUP before scanning for available clear frequencies.
It's not enough that two frequencies (Ex: 478.246 Mhz) are clearly different numbers, because each has a full set of resonant artifacts that must be resolved as well. Just like tuning forks that will set off vibration in another fork of the next octave - radio frequencies have resonant artifacts that can step on your next mic channel.
The issue is further exacerbated when the mics are not the same brand. "Group 2" on a Shure brand has absolutely nothing in common with "Group 2" on a Sennheiser brand, even if they are operating in the same range.
Each group is an already resolved set of pre-coordinated frequencies known to have no resonant conflicts with each other. The "scan" is simply the second additional step of looking for local signals that conflict with channels within that group. These local issues might be a DJ in the next room, or a local TV station, hence we "scan" to see what from a given group is already in use at that location.
The group we should use may change for each location (city, or state) we play. Group 'c' might be what works well in your hometown, but Group 'A' could be a better choice just 10 miles down the road.
This same rule applies to QLXD, ULXD etc. The best practice starts within the same group, and then we look elsewhere should we need to find more non-overlapping channels.
I still have clients who use older analog mics like SLX, ULXP, ULXS, BLX, PGX, etc, and they can work well - IF proper frequency coordination is followed. I had one school that had a collection of 19 various mics: PG PGX PGXD BLX and QLXD, used in a school theatre production that were experiencing dropouts. Ultimately, after doing an actual frequency coordination - they were only able to use 15 of the available mics because no matter which combination you choose - there is no possible way to resolve more than 15 non-conflicting channels from the available products on hand.
Outside of mechanical failure - the mic system itself is almost never the true problem. This is why cheap mics have such mixed reviews. A cheap mic often has limited features and when combined with poor frequency coordination and busy RF environments conflicts arise very quickly. Users in relatively quiet RF environments will experience fewer collisions and quite possibly roam around the local county or state without issue despite never once changing the group or frequency deployed. In major cities, juts moving a black away could dictate a need for change.
Limits arise more rapidly with analog mics because the channel footprints are also wider than that of digital. You can deploy perhaps up to 20 ULXP or SLX mics (analog) but a QLXD or ULXD system can deploy up to 31 channels (50% more) under ideal circumstances. To expand that you have to then go to wireless workbench or similar software to identify compatible frequencies still viable within other groups.
The new frequency agile digital systems will dwarf that since they will operate across the whole available range rather than scanning specified groups.
