Undershirt tzitzis are complicated. 4 cornered garments have to be mostly open on 4 sides, so if you sew up the sides to make sleeves, the majority underneath needs to be open.
Zilzul is another issue, because the undershirt will be very sweaty, especially by its main clientele – teenagers who play ball and run around the whole day.
Some permit this, but it’s not for bnei torah.