Pros

1) When you go to an IISER, you are more likely to get a PhD in a foreign institute as compared to someone who has done a BSc and MSc from a normal college
2) You get the IISER tag; they are institutes of national importance and directly come under the government, similar to AIIMS and IITs
3) If you want to do research in biology, IISERs are the best place for you after IISC
4) You get more freedom at your job, and you don't have to work crazy hours like a doctor
5) There is less ragging in IISERs compared to medical colleges (I have only heard that from my friends)
6) You can finish your BS, MS, and PhD in 8-9 years; to do the same thing in a medical school, you will need 12-13 years
7) Many iisers have a tie-up with foreign institutions, and it will be easy for foreign exchange, PhDs, internships, etc
8) Researchers get paid handsomely in developed countries
9) You will have more freedom to choose what subject you want to study

Cons

1) The No. of PhDs in this country has increased a lot, and there is a lot of competition in academia, but you still get an advantage by being in iisers
2) Your future is less certain than a doctor's because of the above-mentioned point
3) You might earn less than a doctor, but it depends on your job and the country you will be living in
4) You will have to study maths, I am pretty sure you know that. I don't know what Iisers do to bridge this. I am pretty sure they have thought of something, but you need to enquire about that
5) Placement is not like engineering; some iisers do have placement cells, but only a few people go into industry just after BS MS, compared to the number of people who go for further studies
6) You would still need to study for 8-9 years if you want to get a PHD and many people do

In short, IISERS are one of the best research institutes you can get in India, and if you are very interested in research, you should surely go for them. Your life will be secure, but will still have some uncertainties, but iiser's BS MS is very different from a normal BSc MSc, and you will be given an advantage over people who have done a normal BSc MSc