Hi,
I'm just starting out on real estate investing. I've put significant chunk of my monthly pays into stock market/mutual fund since I was in college. I just went into contract on my first investment property last week, so i'm almost at 100% stock (Have small money in bitcoins and other "alternatives" if you count those). I intend on increasing my allocation to real estate over time (without selling out of my stocks/mutual funds - they have significant unrealized gain so it will trigger a pretty large taxable gain if i sold them).
To answer your question about financial advisers, its easier for common people to invest in mutual funds and the stock market. it's more liquid, and all you need to do is buy a mutual fund. not much research required. you just set it on auto pilot. with real estate, you need to put in more effort into finding the right property, crunching numbers, communicate with vendors stake holders, etc. there's a lot more work.
In short, stock market is easy money (in a longer term) and you can do it with smaller sum of money (or at least that's the perception). real estate requires effort, illiquid, requires larger capital upfront, but if done well can do very well. Major advantage point of real estate is taxes. with stocks/mutual funds, unless they are in ROTH IRA or ROTH 401K, you are taxed on the realized gains. You can also invest in traditional IRA and 401K which will DELAY your tax payments, but that's not the same as tax avoidance. For rental properties, you can avoid most of the tax on your rental income for about first 10 years or so from just depreciating your home + interest payments (if you finance your purchase). After 10 years, you can do a like kind transfer and get a new home without paying tax on the gain as well (and then your first 10 years of rental will be tax free again!). I'm not as knowledgeable about flips, but I imagine there are many ways to get around paying taxes on it as well (or at least delaying them).
i don't think if you are doing well in real estate you need to follow 90/10% rule at all, but i do recommend you look into investing a little into the stock market, at least to max out your IRAs and 401Ks (especially, if your employer provides 401K match, you should definitely make sure you get the full match, as that's free money). I also recommend that if you do decide to go 100% real estate, make sure you have some rental properties. Flips may be lucrative and good to build capital, but do you really want to be spending time in your 70s and 80s looking for properties to flip? instead, if you have sufficient rental properties to have enough passive income to cover your expense, you can live freely doing whatever you'd like :)
That was long, but hopefully it'll be helpful to someone!