I think Riverside is a good market for rentals, I am a little bias as I have lived here for over 40 years and really do love Riverside. Riverside is a centrally located area of So Cal and is an area that is mostly made up of Blue Collar workers that can't quite afford to live in OC or LA and come to this area for lower cost housing. It has a fairly robust economy, with the exception of high tech and medical jobs, with a large shift in the last decade to being a logistics hub for So Cal due to it's central location (the major HWY arteries to the rest of the county run right through the IE) and has some of the largest concentrations of large warehouses rivaled only by New Jersey, the median income is a little below the Cal average by about $10K and about $5K higher than the national average, but, it has been growing. Prices are rising, but, for So Cal it is still one of the last "affordable" areas for residents, rents have been strong and demand is high. Riverside has historically been hit hard when RE markets turn down, but, this was due to the fact that the job market was not as robust for the blue collar workers and a large segment commuted to OC and LA and moved back to were the jobs were, this has changed over the years and with more blue collar jobs offered locally we should (cross your fingers) not be quite as bad during future down turns. Another thing Riverside/San Bernardino has that OC and LA do not have is LAND, we have plenty of room to grow our residential and commercial sectors and is one of the reasons prices have remained 'affordable' over the years.
As I mentioned, we have a lot of colleges and higher education opportunities, this is a good rental market to tap into, with areas like La Sierra (La Sierra University), Ramona area (Cal Baptist), Magnolia Center/Wood Streets area (RCC and University of Redlands Riv campus) and University area (UCR) all being good locations for rentals if you are looking for a rental for student housing, some of the Universities will even contract with you directly.
If your demographic is white collar rentals La Sierra Hills, some parts of La Sierra South, Wood Streets, Woodcrest, Orangecrest, Canyon Crest, Alleandro Heights, Sycamore Canyon area, Mission Grove, some parts of Downtown area as well.
If your demographic is blue collar, Downtown Riverside, Wood Streets, Arlington area, Ramona, Magnolia Center area, Sycamore Canyon, Orangecrest, Woodcrest, some parts of Downtown area, La Sierra (Hills and South).
If you are looking for units, Downtown Riverside area, Eastside (but it is hit and miss), University area (also hit and miss), Canyon Crest (hard to find), Ramona area, La Sierra, Arlanza (hit and miss).
If you are looking to go outside of Riverside city, I would look at Perris, Perris is rapidly becoming a logistics hub with a large Amazon distribution center with several large name companies having the same in the area (Lowes, SEKO, Walgreens, Deckers, Ross, General Mills, Hanes, Wayfair, Starcrest, Home Depot, Clayton Homes. Whirlpool, ect). Prices are on the rise and still in the affordable range and it has been a growing area. Also Murrieta and surrounding areas are also a very desirable area and will have a good demand for rentals for blue collar workers.