The UA variants are neat to look at.
I've found that many DMs are willing to compromise if a player is willing to resrict. For example, the UA variant that sugests that conjurers always summon the same creature with a certain spell is one of my favorite compromises. I'll say to the player at character creation:
"I'll let you offer up a few alternative creatures that are within the CR guidelines established by the charts for each level so long as when you cast the spell you always only summon that creature." [Or that creature and its same type buddies if using a spell higher to get more.]
I like this variant because it allows me to be able to tailor adventurers in two ways. First, I can tailor adventures so that the summon spells are immensely useful. Hence, the player feels like that spell was worthwhile. But, I can also use that knowledge for my smart bad guys, too. They'll learn what the conjurer can summon and will likely occasionally have ways of dealing with it. This lets me challenge the player, too.
I really like that variant, top of the line. Plus, I feel like if the player is willing to concede and only summon one type of creature with the spell, I'll let them stretch the CR boundaries a bit. I never let them use a creature whose CR is beyond anything else on the list, but there are some creatures of equivalent CRs that are worth this trade-off. So the players can get a small boost in power/effectiveness and the game gets streamlined.
I do let spellcasters who cast spells out of books scribe a summon spell of a certain level more than once under this variant - each time the spell relates to a different monster. [In this case I encourage them to name each spell something like Summon Unicorn, Summon Dire Wolf, etc] But they have to have each spell they desire to cast in their spellbook. Plus, this variant is kinda cool when handing out treasure. You find a scroll of Summon _____. Players get excited over this type of thing when using this variant. I also let spontaneous casters with this variant add it to their spells known more than once - each one pertaining to a different monster. I don't get many who do this, however. Spells known and spellbook pages are valuable.
