For example, sometimes I have a calm webcam stream open with animals eating from feeding trays while i'm doing something else on the side, and then suddenly without warning, i'm jumpscared by an extremely bright (for example very bright pink with flashing lights) and loud gaming ad.

This makes the advertisement and viewing experience that much more frustrating and for viewers with sensory and similar issues, can be extremely overstimulating and possibly in extreme cases, could be even bordering on painful. All these factors together, I don't imagine frustrated and/or overwhelmed viewers will be likely to be interested in the advertiser('s product) and may even purposely avoid them in the future out of frustration.

Some way(s) of making the transition smoother and less abrupt would help alot. An on-screen timer for all users, not just mods and the streamer, would help, but maybe also have it fade into the ad somehow, both visually and audibly instead of just instantly playing, perhaps with settings to made that fade time longer or shorter (maybe even being able to completely remove it if someone wants to).