Why builders are not given possession to alloties

I am searching for a 2 bed, d/d flat in new karachi (nagan chowrangi onward) area.

I noticed many projects are in finishing condition but builders are not handing over possession to the buyers.

I am particularly interested in 2 projects 1. Hani's excellency, 2. Saima arabian villas, both projects are in finishing condition and builders is saying if we make full payment now then they will give possession after june.

I am worried if there is some hidden glitch in this deal.

Please share your expert opinion.