polymorph at the levels where you first gain access is broken. at level 7, you can use a spell slot to turn someone into a CR 7 beast (my recommendation is the giant ape for CR 8 as well, having a ranged attack is very nice). you gain a huge amount of hit points that you can throw away over the course of the next hour, your damage is vastly out of line for what you'd expect from someone at that level, you gain basically the same athletic ability as a proficient warrior at that level, and you can do things with it that a proficient warrior couldn't hope to do (like grapple a creature larger than large size). and this can be applied to any character a wizard who might need to spend resources to contribute to several fights can simply spend only a level 4 spell slot and be able to contribute about as much as any primary melee class for the next hour, but doesn't have to care about HP lost in the process.this is in addition to opening up a world of utility options, of course.
this is relatively speaking not a problem for long, but only because there are no beasts above CR 8.
note that for a group of 4 level 7 characters, a "deadly" encounter would be an exp budget of 1400 exp. a single CR 7 creature is worth 2900 exp, or over double that amount. or, in other words, you can turn a character into something massively more dangerous than the hardest recommended encounter for an entire group of that level.
recommended solution: i think it might be best to limit the spell to CR 5 (one higher than summoning spells because you're replacing a character rather than adding a new threat), plus one per additional spell level above 4th. however, i have not playtested this solution admittedly.
edit: correction, as noted below after the pixie rant: the encounter budget is per character, so it isn't quite as bad. a single CR 7 is a deadly encounter for 2 characters, however; that is, when outnumbered 2 to 1, it is expected that this creature will have a good chance of killing one or both characters.
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conjure woodland beings can be used to obtain 8 pixies which can cast a large variety of spells, including the above mentioned polymorph, confusion, sleep, and a few others.
recommended solution: pixies should be higher CR. personally, i think CR 1 would be best; 2 pixies is still a good deal in some ways, but not nearly as destructive to game balance as 8 from the spell, and the fragility of the pixies means that it is still hard to make good use of all their offensive spells without the pixies getting killed.
as an added benefit, this also fixes the problem that CR 1/8 pixies that do fight the party for some reason or another are going to crush the party mercilessly if valued accordingly. consider, for example, the sleep spell. a single pixie is *supposedly* an easy encounter for a level 1 group of 4 characters. on a good roll, the pixie can put 3/4 of the party to sleep with no save (on a *really* good roll, the entire party may be put to sleep with no save. note that the pixie can still be invisible at this point). 2 pixies (supposedly a "hard" encounter) is an almost-guaranteed TPK as the pixies can most likely put 2 people to sleep each. in either case, if the party has sustained even minor injuries in a previous fight, their chances of being able to act at all decreases substantially.
consider, for example, a typical first level party. for the sake of argument, we'll give 14 or 15 constitution all around, and make it a fairly typical wizard, rogue, cleric, fighter party. an average roll for a pixie is 23.5 HP for a sleep spell, or enough to easily take out the wizard and either the rogue or the cleric. a reasonably good roll (28 or higher, not unreasonably hard to roll at all) will take out whichever of the cleric or rogue are awake. from 2 pixies, a second sleep spell will, on average, put the fighter and cleric or rogue (whichever didn't previously fall asleep) to sleep. assuming the first pixie didn't roll well.
as noted, if each party member has sustained 2 HP of damage in previous fights which they have not healed up, their chances of staying awake decrease dramatically; specifically, a roll of 23 (just a hair below average) on the first spell will put everyone except the fighter to sleep with no save allowed.
this is without even touching on the potential to open up with one of their spells that can hit the entire group and only allow saving throws at what will be about a 50% chance to save. at CR 1/2 a single pixie is considered a deadly encounter (as might well be expected from a monster that has a good chance of putting the entire party to sleep in the first round with no chance to react). at CR 1, it is double the XP of a deadly encounter (bearing in mind that the single pixie can open with turning the fighter into a creature with 0 CR, and then drop a sleep spell that suddenly has a very real chance of putting the entire party to sleep even if none of them are wounded, and is almost guaranteed if they were all hurt a little bit sometime during the rest of their adventuring day).
for contrast, consider the stirge (another CR 1/8 creature). with AC 14 and 2 HP, and about a 50% chance to hit a level 1 character that cares about AC, there's a significant chance a single stirge will do nothing before it is killed. this is appropriate for what is described as an "easy" encounter.
so yeah, CR 1/2 or 1 pixies should really be a thing.
edit: correction, i've just discovered i was reading the encounter rules wrong. encounter budgets are far more generous than i was reading; a pixie is supposedly an *easy* challenge for a *single* character, not a party. the XP budget for 4 characters considers 2 CR 1/8 creatures to be less than an easy encounter, and 3 to be somewhere between easy and medium, closer to medium (4 is a "hard" encounter).
or, in other words, it is even worse than i originally thought.
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edit: almost forgot, any item or class feature that boosts save DCs (possible exception if there is a massive and immediate cost). by the time high levels are reached, many monsters will have +5 to +10 in their good saves, but only +1 or less in their bad save(s). meanwhile, expected DCs for characters will be 19 (8 + 6 proficiency + 5 attribute) at the highest levels.
the result is that on a d20, under normal circumstances, only monsters with a -1 save or worse will require a 20 to make their saving throws.
now throw in a +2 DC from a robe of the archmagi or staff of the magi (or a +2 rod of the pact keeper, or a stat-boosting book and the ioun stone that increases your proficiency bonus). suddenly, you need a +2 bonus just to avoid dropping to a flat 5% chance. heaven help you if your caster manages to get their hands on several of these boosts (the *absolute* worst-case scenario is of course quite improbable; a warlock with 5 stat tomes, a +3 rod of the pact keeper, and both the robe and staff can get DC 31 spells, which makes even a +11 save modifier from a monster basically irrelevant).
and remember, these saving throws can massively alter how a fight goes; even a lowly entangle spell can turn a tough fight against a mob of assassins into a joke if the DC is high enough.
