Are there any downsides I may be overlooking?

@Jose Jimenez I'm not sure there are many downsides, other than dealing with roommates.

Could do a construction loan then refinance the property to pay off the construction loans.Junior ADUs must be attached to the primary residence (i.e - attached garage can be converted to a jADU, but detached garages do not qualify) and require owner occupancy which is a huge downside if you are trying to buy a new property every year for owner use to get away with <20% down payment.For some numbers, my main house is 4bed/2bath renting for $3500/month and ADU 2 bed/ 2 bath + Den used as 3rd bedroom will rent for around $3000/month.

The seller may say the land is "perfect for building, with several building spots," but does he really have any idea about the downsides of his property, such as how much getting utilities to it will cost, or the percent of it that is unbuildable?

You most likely are looking for what's called a DSCR lender.This lender (typically) won't look at you as a borrower but will instead solely focus on the property itself, how much free cash it can generate, and if that free cash is enough to cover the debt.The downside is they will (typically) charge quite a bit in points, be at least 100 basis points higher, and can be slow to closeBest of luck!

I work a full-time job, so I'd be limited to nights/weekends.Are there any downsides to becoming a licensed agent, as an investor, that I may not be aware of?

I just wanted to open your eyes to some of the downsides that aren't always addressed in the books, podcasts, and forum posts that inspire people to get into house hacking.

There is no right way to get there and it’s not a race.You should only tackle a project you know you can confidently handle and never raise capital until you’ve done a deal with your own money and understand the up and down sides to a deal.Cheers to your success!

You mentioned that in a bad year you break even so the downside is limited. 

The downside is there is only one electric meter/panel for the whole house and there would be one bill for both units which includes heating/Central AC/electricity and so forth.