They're interesting neighborhoods - old fashioned, blue collar, and traditional in their own way. You'll find accommodation and food cheaper than in the glitzy parts of town, which makes these areas attractive to budget conscious tourists. They might not look as tidy as recently developed areas, but they're safe, clean, and full of character.

Ueno is one of the five or six hubs that encircle Tokyo, so it has great railway connections (including the Shinkansen lines to the north and the Keisei line(s) to the airport). It has more of a "big city" feel, but there are nearby neighborhoods like Yanaka that give you a sense of a bygone era. It's handy for several national museums in Ueno Park. Asakusa is by the river and sees a significant number of tourists who visit Senso-ji (an impressive temple complex).

Plenty of foreign tourists stay in both areas. Since Tokyo has an excellent transportation network, they're both fine as bases to explore the rest of the city.