With a new rewiring of the stock electrical system, as long as you do it right, your build will be just as electrically reliable as it was out of the factory. The biggest "killer" of these old motorcycles in electrical terms is corrosion at the terminals, especially ground terminals that meet the engine and frame. It's on you to ensure every connection you make is solid, vibration-proof, and resistant to corrosion. If you do those three correctly at every single connection and perfectly replicate the original wiring diagram (where you haven't changed it with a new gauge setup) you'll have no wiring problems.
There's nothing inherently unreliable about the original wires unless they've been physically damaged, as far as I know. They're stranded copper wire just like anything else. They'll oxidize just a bit where the copper is exposed but that can be cleaned and polished off. I bought a used complete wiring harness for my bike from 1980 off of Ebay and tested it with a multimeter. Every connection checked out perfectly. There was a bit of patina on the terminals here and there but the multimeter said everything was good with low to no measurable resistance.
The X21 Plus kit you mentioned comes with a set of handlebar controls that look like cheap junk you can get for $10 on Ebay. The unit itself looks like one huge expensive integrated circuit that you buy to avoid learning how to do wiring -- sure, it may make wiring the whole bike yourself from scratch easier, but diagnosis if you have issues down the road becomes way more difficult. If it were me I'd save my money and learn how to read and replicate the factory wiring diagrams. $300 can buy more than enough wires and electrical tape to rebuild a complete harness and that unit doesn't exactly look like my idea of quality construction.
I haven't installed an M-unit but any univeral gauge unit will have its own install procedure and they're all pretty similar. Here's what I ended up doing with my own machine.
I replaced the stock cluster on my 1980 CB750K because when I opened up the outer cover of the original gauge cluster, I found that every piece of plastic in the cluster (every part of the cluster except the support bracket basically) was cracked in some way. Some were cracked in multiple places. I didn't trust it to keep bugs and moisture out of the original mechanical gauges, or even stay on the bike for very long, with so many cracks.
I first bought a pair of "mini" chrome-housing gauges, one speedometer/odometer with integrated warning lights and a tachometer to match. I think they came from 4into1. Both were cable-driven mechanical gauges. The odometer broke after 1.5 miles and I confirmed it wasn't the cable. Cheap Chinese junk, do not recommend.
After that I decided to pay up and bought a Koso TNT-01 multifunction gauge, which was about $200USD. It involved a good amount of custom wiring and some custom lathe work to replace the stock speedo drive unit and make a hideaway magnet solution for the new electronic speedometer sensor. These two steps were completely unnecessary and were done for cosmetic improvements. The cheap but functional solution is to epoxy the sensor magnets onto the rim of your front wheel and zip tie the sensor to the fork leg, and leave the stock speedometer drive gear in place.
The custom wiring involved taking every wire in the headlight bucket that went into the stock gauge cluster, separating them from their plugs, and soldering them into new plugs and a couple barrel connectors. Did the same thing on the wiring side for the new gauge. Had to match up the wiring diagram from the motorcycle harness with the one for the Koso gauge and that whole process took several hours of work, plus a couple sessions of correcting mistakes. There was one difference that I had to make up for between the Koso and the stock gauge cluster: the stock cluster had individual left/right turn signal indicator bulbs but the Koso only has one for both circuits. I had to solder in a diode, on a Y-pigtail, between the two positive wires for each turn signal indicator and the positive single wire for the Koso's.
If you are a total beginner to automotive/motorcycle wiring, the absolute best thing you can do is have someone who is experienced help you in person. There are plenty of tutorials online and I encourage you to read/watch plenty before attempting but there is no substitute for experience.
