I agree the system could be simplified somehow but for me it seems there's nothing wrong with the chances. First melds are always 100% and you don't really need anything more than that for any content. AF + GC gear + single melds = pretty decent and cheap gear set for any job.

If you want to go for double (tier III-IV) the chances are usually around 15-25% (those are from what I've personally done, not sure about highest IV+IV). That's really not too bad for that powerful stat increase. You can count the average risks and costs: if you have a 25% chance to succeed, you should be prepared for around 4 tries and not expect to succeed on first try.

I haven't done any extensive testing on melding, but I've done several double melds for myself and some LS mates and I just recently did a triple melded piece for myself. When I started the process I checked that the chance for first meld is 24% and for the second one it's 9,39%. I calculated that approximately every 4th of the double melds would succeed (in the end 15/43 = 34,8% did) and 11th triple meld could succeed (in reality 15th did). The combined success rate should have been around 2,25% so I prepared for 45 tries. I succeeded on my 43th. In the end, a triple melded III+III+IV piece ended up costing me 5,3M which is definitely not too expensive for such a huge upgrade. And no, I'm not super rich, I just don't feel like this game should be throwing amazing gear at me for free either.