CapnZapp
Legend
A Cleric 10/Druid 10 will have high level spell slots, but not high level spells.I can't under stand your explanation on some of this but a Druids 10 / Cleric 10
PHB: p164
Spell Slots
"You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes."
Druids 10 + Cleric 10 = level 20 multi-class caster on the table
Spells slots of a level 20 multi-class are identical to a full Druid or Full Cleric at level 20 (There is no loss)
Known and Prepared Spells
"You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class."
(Assume 20 wisdom at level 20)
Cleric lvl 20 Prepares 25 : When you do so, choose a number of cleric spells equal to your Wisdom modifier + your cleric level (minimum of one spell). The spells must be of a level for which you have spell slots.
Druid lvl 20 Prepares 25 :When you do so, choose a number of druid spells equal to your Wisdom modifier + your druid level (minimum of one spell). The spells must be of a level for which you have spell slots.
Druid level 10 Prepares 15 + Cleric level 10 Prepares 15 = Total of 30, so you get 5 more than ether class at 20 but they have to have 15 on each side, which makes over lap is good for flexibility on key spells.
For "known spells" you have access to all Druid and Cleric spells since you have all the way up to a 9th level spell slot per how those to classes work. Because they "Know" all spell inherently.
All that said you are correct in that they do get access to every thing later. The even out at levels 8 and 16 and suffer the lose of the level 20 feat/ability score. However, human variant could help you get the feat back up front which means you get more use out of it. Then your just short capstone abilities in exchange for a huge amount of versatility. A Circle of the moon druid for example would suffer greatly however a Druid from the Circle of the Land would not lose much mechanically. You also open up some options like Good Berries on a Life cleric for 30 hit points of guaranteed out of combat healing from a 1st level spell slot per Sage Advice. ( http://www.sageadvice.eu/2014/09/11/discipline-of-goodberry/ ) So if your want to be one of those healers that only uses heal word in battle and doesn't heal between this gives a cheep option to heal your team between battles if they start getting annoyed by always being left a deaths door. (Which I hate when I am not a healer. Also, if your going to do that take the healer feat so you can still patch player up a bit if you don't want to use your precious spell slots. Just Saying.)
I have not idea what you mean by this --> (and they lose out on the cool level X+1 to 2X abilities) if you could clarify, I would appreciate it.
You can still use your 9th level slot, but only to upcast your 5th level spells (since that's all you have).
And this is seen as significantly less good - the power of upcasting in 5e is not nothing, so it's way better than 3E caster multiclassing... but it is considered significantly less powerful than the ability to cast "real" level 9 spells.