ADL5502 has a low frequency boundary of 450 MHz according to the datasheet. What would be the issues if I operate it around 100MHz? Any hint?
This low-side frequency limit is caused by an ac-coupling cap in the main signal path. So a 100 MHz signal will be attenuated before it gets to the detector. In Figure 5, you can see the effect of this starting to kick in. Notice how the response at 450 MHz (blue) is sightly lower than the response at 900 MHz (pink). At 100 MHz, I think that you will still get a response but its slope in V/V will be much lower.
A good alternative for you may be ADL5511. This has rms and envelope outputs but no peak-hold mode.