I use Yes Bank, so to answer your queries :
1. Yes, OTP is required. Apart from SMS OTP, Yes Bank offers other OTP Delivery methods as well
2. 3 PM Cut-off time for both categories, retail and HNI : SME & Mainboard both
3. Interface is good, no major issues. Unblocking also happens without any significant delay, at least for me it hasn't disappointed yet.
4. Yes Mobile App does show E-IPO invest feature, especially in the Yes Prosperity account variant. But at times for some users that option seems to be missing.
As for not working issues or downtime, that can happen with any of the banks as there are no exceptions to it. Just that frequency might be more or less in some banks.
@Sumanta De