Air BnB’s vs hotels in Barcelona, Madrid, Sevilla

Since you're asking that question here on these particular boards, you should know that If Rick were answering your question, he'd probably say that you should avoid using short term rentals like AirBnBs because they gobble up housing for locals and ultimately destroy the character of the neighborhoods that we are seeking to experience. He has talked about this in videos and blogs.
Others would say that the genie is out of the bottle and locals certainly don't mind having buildings fixed up by investors.

I like small hotels and historical places so that's where we try to stay. If we find a chain that has the vibe we like, we stick with that a bit.

As far as rentals go - I tend to miss cooking after a couple weeks, so we will often break up a long trip with an apartment. I look for a kitchen apartment that is clearly someone's extra apartment; a room above their restaurant or an apartment that's empty while a family member is working abroad. It may seem weird but I avoid places that look like they just got a refresh; brand new appliances, white, beige and grey tones, metal & plastic furniture, etc. I look for places that have family knick-knacks, clutter, unmatched furniture, actual artwork on the walls.

In Spain, you have an awesome system of historical hotels that function as hospitality and culinary schools while preserving some spectacular properties. https://paradores.es/en. The last place we were at was hosting a large number of EU members awarding some honor to the Spain delegate. We paid less than €200 for that kind of place and the meals were really fantastic. Sign up for a free membership and you'll get better pricing; especially on stays 3+ days.

Like you, I am an AirBnB user (and former host). Barcelona is a city that, were I to return, I would not use the platform there, as it seems to have overtaken the local housing market. FWIW, other cities where I have moved away from the platform include Dublin and Amsterdam. Instead, I look for purpose-built apart-hotels. They have a lot of the sterile qualities mentioned in the previous comment, but I like to have my coffee in my PJs and to sit on something besides the bed. I do like the other commenter's idea of only considering places that appear to actually be lived in. I have certainly had the best stays in homes that were vacated for me, as they usually have sharp knives and good pots and pans to cook in. And the olive oil in the pantry isn't rancid.
An apart-hotel chain I have read about on the Forum is Citadines, which has a location in Barcelona: https://www.discoverasr.com/en/citadines/spain/citadines-ramblas-barcelona?utm_source=google&utm_medium=maps&utm_campaign=hq-google-maps-alwayson-20240531-all-all-eu-es-citadinesramblasbarcelona-yext-gbp

Definitely apart-hotels bridge the gap of short let apartments and hotels with no kitchen facilities. You can certainly do a mix of lodging types as well. You don't mention whether you plan on driving or train (which is very good in Spain) nor if you prefer cities or rural. If mainly cities, cars are just a pain and finding parking is pricey and another headache.
If you decide to do Airbnbs just ensure they provide their registered license number, particularly for Barcelona. Any reputable highly rated host will just include it on the listing.

Air BnB is hardly operating in Spain, since the legal conditions are so onerous.
Go for a properly licenced aparthotel.

We stayed in dedicated aparthotels in both Madrid and Sevilla in November. You can read about both in my trip report here:

https://community.ricksteves.com/travel-forum/trip-reports/trip-report-analucia-and-madrid-in-november

The one in Seville was recommended in the RS guidebook. The one in Madrid was recommended by a forum member. He recommends the apartments with views of the royal palace.

Over the years, I've used Airbnb less and less. Even after researching a property, other issues can arise. Some owners are more motivated than others to resolve problems. I prefer dealing with a front desk. I think aparthotels are the better choice. I have stayed in Citadines in both Germany (Munich) and France (Toulouse and Montpellier). It is a solid option. Every room includes a kitchen.

One of the reasons we prefer AirBnB is that, if one rents from individual owners, one usually finds oneself in a local neighborhood with natives congregating and the "local scene" in evidence. Certainly we had that experience last summer in Madrid, Valencia and Murcia. Honestly our experience in Sevilla was so forgettable I don't even remember whether it was hotel or AirBnB.

I fully agree with the comment above about spurning AirBnB in Barcelona. Clearly there is a housing crisis in that city that dwarfs what's going on in other Spanish cities. But then, I'm never going back to Barca for any reason.

I figure the locals are going to come closer to deciding correctly what is in their best interest than i am or anyone who isn't a vested member of the local neighborhood and culture is. Mama used to say that the road to down-under is paved with good intentions.

Based on that I chose what fits my needs best, that is legal.

I stayed at an apartment owned by DESTINATION BCN. I've recommended the company to others who were also happy with the rentals. Look at their offerings in the building near Universitat/Plaça Catalunya.