Tenser's is also hidden away in Xanathar's. I was not even sure it was in 5e at all.
I would hesitate to say the supplemental spell should be used as a baseline to measure core spell appropriateness and that there are not problems with Tenser's Transformation's power balance.
For those without Xanathar's:
TENSER'S TRANSFORMATION
6th-level transmutation
Casting Time: 1 action
Range: Self
Components: V, S, M (a few hairs from a bull)
Duration: Concentration, up to 10 minutes
You endow yourself with endurance and martial prowess fueled by magic. Until the spell ends, you can't cast spells, and you gain the following benefits:
• You gain 50 temporary hit points. If any of these remain when the spell ends, they are lost.
• You have advantage on attack rolls that you make with simple and martial weapons.
• When you hit a target with a weapon attack, that target takes an extra 2dl2 force damage.
• You have proficiency with all armor, shields, simple weapons, and martial weapons.
• You have proficiency in Strength and Constitution saving throws.
• You can attack twice, instead of once, when you take the Attack action on your turn. You ignore this benefit if you already have a feature, like Extra Attack, that gives you extra attacks.
Immediately after the spell ends, you must succeed on a DC 15 Constitution saving throw or suffer one level of exhaustion.
The possible exhaustion at the end is a big drawback, it does not seem that necessary to balance a 6th level concentration wizard buff to turn them into a temporary OK warrior who can't keep casting spells during the duration.
I'm...not sure this argument does what you intended. You seem to be holding it up to say "maybe
Tenser's transformation isn't a good example of balance" and then citing a factor which
further limits its power.
That is, you seem to be saying this in the sense of "maybe TT is weak?" And all I'm seeing is "TT is actually
even more restricted than we thought," making an even stronger case for
polymorph being over the top. A sixth-level Wizard-only spell (so, only cast able a couple times a day maximum, because no way are you casting this with higher spell slots, and not usable by most classes), which gives comparatively mediocre benefits (basically makes you a weaker Paladin with no spells but always-on advantage on attacks), only lasts 10 minutes, and when the spell ends you (very likely) take a level of exhaustion. And you can only use it on yourself.
By comparison,
polymorph is 4th level (so you can cast it many times a day), available to most full casters, provides arguably better and
definitely more versatile benefits, can be cast on other people (so the "no casting" drawback is moot), lasts a full
hour, and not only does it
not inflict lasting consequences it in fact cushions the blow by making only excess damage carry over if the spell ends early due to being reduced to 0 HP.
I'm legitimately not seeing
any way in which
polymorph is appropriate for a 4th level spell at this point. And the idea that we should
power up casters because
polymorph is "only" 4th level would be hilarious if it weren't deeply saddening.
Who complains about druids turning into bears? I wouldn't play a druid that couldn't Beorn-out every one in a while.
Oh, plenty have complaints about it. Usually centered on the "Moon Druids are unkillable because they have huge stacks of HP in animal form and dropping them to zero just pops them back into human form!" I can't
entirely fault these concerns, given how absolutely and ridiculously OP the Druid was in 3rd edition, but yeah there are much more serious issues than 5e Wild Shape. That's like number 25 on the balance adjustment priority list. It's
there but there's a couple dozen other things that need attention first.