Tenser's seems weak. Pointing out a weakness of it seems to support the argument that Tenser's might not be a good benchmark of power and effectiveness of 5e spells in general.
I don't know that Tenser's stands up to say Mass Suggestion for 6th level concentration wizard spell combat offensive usefulness.
4th level polymorph should probably be compared to other 4th level spells. A good point of comparison would probably be the core PH wizard 4th level concentration buff spell Greater Invisibility.