You don't really "need" to upgrade your units if you want a semi-competitive setup in general. I think most minions are best at base anyway, with only some big exceptions like RoyalGuard++.
This is an estimate, it's been a long time since I first started, but challenges give you 15 rubies a day = 450 rubies a month = 9 ultra crates a month, or you could convert them to 1.3 million coins. It might sound strange that I'm focusing on the challenge rewards but getting 15 rubies a day from PvP matches would take a lot of time in that day.
Due to basic statistics what will happen, at least if you buy crates, is that you won't get +++ legendary for a long time, but once you get your first +++ legendary, the next will be much shorter to get, basically a wave of +++ legendaries will happen. This truism applies for all rarities and tiers but it's not as noticeable. Or you can get +++ legendaries directly but the 100+ ruby costs are pretty ridiculous and would take months to get.
Because of this I think it is significantly not worth it to specifically buy a +++ common or rare. Packs are going to get you almost all the way there anyway in about 2 months for both those categories, but if you are impatient, you can just buy one directly in less than a month (like a week or two). The cheapest epics (10 rubies) can be direct-buyed in like 3 weeks, I suppose, and the cheapest legendaries (40 rubies) direct-buyed in a month. You'll have to multiply the price accordingly though for more expensive stuff, the 200 ruby legendaries take 5x as long.
Keep in mind again that you should have a large collection of units and some playable competitive army before doing direct buys because saving ruby for them means no progress for any other units.
Thank you really much for the very detailed answer! Challenge rewards is a great suggestion. We are talking about 450 rubies or 1.3 million coins a month... could I ask you if, for long term purposes, it would be better to buy all ultra crates possible or to convert rubies to coins and buying like thousands of normal pack? (at this moment every ultra crates is worth around 170 000 coins, it's around 17k coins for 5 rubies, I don't know if it changes a lot or what is the average). If it's better to buy only ultra crates so I'm doing right, because these days I've been converting every coin I got into rubies to buy ultra crates. I felt it was more likely to make give me higher rarity units, and that it could be better on the long run, Thank you again!
According to your exchange rate 0f 17k [coins]=5 [rubies] an ultra crate costs 17k/5*50/1000=170 basic boxes
So first of all it's obvious that basics increase your unit pool way faster.
The rarity occurences within the packs is hidden. It's very certain tho, that Ultra crates chance for Legendaries is less than 10x as high as for basic packs.
https://chess-evolved-online.fandom.com/wiki/Shop
https://steemit.com/hive-135459/@e3gewinnt/empirical-rarity-occurrence-in-basic-packs
So to be better than basics to gain Legendaries Ultra Crates would need to cost less than 10x as much as basic packs.
The chance for Epics pretty sure is less than 5x as good in Ultra Crates compared to basics, so to be worth to get Epics Ultra Crates would need to cost less than 5x as much as basic packs (On the short run Ultra Crates have the gimmick tho to garanty an Epic unit).
So rn Ultra Crates are pretty bad.
mmh I understand even if, for my limited experience, probabilities were a bit different. Maybe I've been extremely lucky but in every ultra crates I opened so far (they should be like 6/7 in total -consider the new player bonus, that I don't know how it worked but surely, while it lasted, gave me a lot of coins), in every of those ultra crates I've found a minimum of 1 legendaries, to a max of 4 if I remember well, for an average of 1.5 or even 2 legendaries for crates. I repeat, maybe I've been extremely lucky, but that's around 1.5 legendaries for 170k coins, against the 1 legendary for 200k of your stats (because in the post about the 12000 basic packs you talked about 0.5% legendaries chance). For the moment I'm wondering if it's just luck or something, so I've just decided to continue to buy ultra crates -for scientific purposes- and let you know how the statistics change.
Grand confirmed, that the first appearing Diamond boxes are rigged positive to be choosen 4 sure, idk if such positive rigg also applies to the earlier buyed chests (Probably this is wrong speculation by me).
(I Edited comment some times, because I confused myself)
I'm definitely interested in your future results in Ultra Crates anyway.