I have gone down the Ergo rabbit hole and escaped relatively unscathed. Pointy hood Ergos are a 1992-1998 and were all 8-speed except for 1997-1998 (Record 9-speed) and 1998 (Veloce and Athena). Crossover to the rounded hood Ergos occured in 1998 as Record was redesigned with the new shape for that year (9-speed still). Both 9- and later 10-speed used rounded hoods from 1998 and on.

There is a difference in cable pull for 8s Ergo, "old" 9s (aka pointy hood) Ergo, "new" 9s Ergo (round hood), 10s, and 11s. Shimano does this, too.

For rear derailleurs, the cable pull ratio is simpler: "Old" which is 8s era and "old" 9s era -- "New" which is "new" 9s era to now

This webpage is 1000% percent dynamite for helping out with cable pulls and mix-and-match calculations.

Sheldon Brown's Cassette Cog Spacing is also gold. To be used in tandem!

The hubs and splines are different, kinda like Shimano's Uniglide and then Hyperglide (what we've been using since 1990) only more inconvenient. Employing Shimano cassettes/cassettes with Campy spacing helps, unless you have killer used bike shops close by as you'll be in much better shape then due to selection.

As for internals, all Ergos up until 11-speed use side-specific G-springs and side-specific G-spring carriers. Other than left or right designation, G-springs are the same from 8s through 10s. The part numbers for these internal bits are the same from the low end to the high end, with the exception of Carbon-suffixed Ergos (Record, Chorus) with it's plastic carrier (vs. aluminum for everything else). Heck, even the hood part numbers are the same unless ("Carbon BB" model or "Escape" (single) shift excepted--same outer hood shape, different internal molds). To me, it seems like one is paying only for different letters printed on a brake lever arm. Wow, ok, lots of small exceptions, BUT pretty much, most everything is the same inside.

You will know the Escape shift by the thumb lever only having a little slot to work in the brake hood. The "full feature" as I call it has a huge slot in the brake hood where the thumb lever can travel 120° of rotation to upshift the entire cassette. Always standard on Record and Chorus, in the mid '00s that feature left the lower end Ergos before returning for a little bit. The new 11s hood shape (some Ergos are still 10s, though, with that new shape) has reverted back to the Escape (lower three tiers) and Ultra or normal or "full feature" for Chorus and Record.

In my few years experience with multiple 8s, 9s, and 10s Ergos, the shift feel has gotten increasingly better. 8s have a dead 'spring back' feel when operating the thumb lever while operating properly. 9s downshifts (big lever) are decidedly longer in throw compared to 10s. The general ratcheting system is solid and you pretty much don't botch a shift like you can with Shimano stuff (aka a partial shift). That's nice, as is the immediacy of the shifting result. Move the lever = shifting right now. Shimano has their ka-click which results in a change a split second slower. I like both feelings for different reasons.

I have found that the easiest way to tell that an Ergo needs rebuilding (if only the aft half of it) is if, when I push the thumb lever to shift up and release, I get a bunch of nasty snappy metal-on-metal sounds and a resulting jolt--easy to tell if levers are off the bike not cabled up. The G-springs (2 per side) and for all intents and purposes, the G-spring carrier, are tired and need replacing. Basically, you tried to shift up 3 gears, and the springs didn't catch their ratchets to hold that top-selected gear, and now slide back to where you started. There are videos and instructions on how to disassemble. Take your time, be patient, breathe(!). It's a battle with small parts, and perhaps a cloth covered vice helps a lot.

Also, and finally for now, that first allen-keyed bolt at the back of the Ergo--on 8s/old 9s (pointy hood) Ergos, they are reverse threaded. New 9s and 10s are righty-tighty/lefty-loosy normal affairs.

Hope this helps!!